1) I noticed when he was colonizing he destroyed seige frigs with his cap, left and then built turrets to kill pirates. Is this how everyone plays?
Some people do, but I think most people kill the entire militia before ordering their cap out. The problem with leaving is that your cap ship won't get the experience from killing the militia. Perhaps if you are in a big hurry to go someplace else or if you just want to colonize a planet and leave the militia around it to force your opponent (who you expect would get it) to have to bomb it (slowing his expansion) you might do that.
2) The player rankings in diplomacy pretty correct? Worth using to assess the situation?
Player records are unreliable because you can make up to five accounts (with their own record) and delete accounts and make new ones. Thus, a player with 1000 games played and 450 wins could play under a name with a record of 2 games played, 1 win, 1 no result under a new name. It's called "smurfing" and it sucks, but that's Sins.
There is one way to feel more confident that you're playing against newer players. Host or join games that are set to the "Normal" speed setting. Most experienced players won't want to play with that. You'll also have a better chance of finding newer players in Regular Sins (and not Entrenchment or Diplomacy) as well as on the weekends.
3) Saw the power of somebody using all the illum turrets today.(If I understand correctly there are 3? 1 front, 2 sides? He parked 'em in the middle of my fleet. Should I have immediately moved my ships back in front of his illums so he couldn't do as much?
You need to determine that he is spamming out Illuminators ahead of time and then build the proper counter--perhaps your own LRMs or a large swarm of scouts (if you are TEC or Advent). You could also make heavy fighting cruisers (HCs)--Kodiaks, Destras, Enforcers, though you'll want about a 1:1 ratio of them to the Illums. Another option would be carriers with fighters. You do want to scout your opponent ahead of time and see what he's building so that you can prepare the counter.
4) It seems in multi understanding enemy fleets is almost more important than understanding your own. Can somebody tell me what the advent colonizer was doing to damage all my ships at once? Is that even what it was doing? Is it a good idea to play all three civs to at least be familar with em, instead of just reading about em?
That's a good insight, and one of the reasons why it's good to be familiar with all three races and to have spent some time playing and studying all three. You need to understand the enemy fleet in order to counter it. The Advent mothership (Progenitor) was using an ability called Malice which distributed 30% of the damage taken by a group of ships to all the ships in the group after ten seconds. The Progenitor also has the ability to restore the shields on itself and nearby ships all at once, making it one of the best capital ships in the game.
5) Can y'all explain feeders to me? Does it depend on the game as to when the feeder begins to feed? Seems as if he waits to feed he feeds better, but if he doesn't feed soon enough why even be a feeder? Tried downloading replays to watch where someone showed each person playing their position, but I couldn't get it. If you can, try to lay out some guidelines for feeding.
If you start out between two allies and thus do not have an opponent on your flank and do not need to fight right away, then you can become a feeder. You will give credits and minerals to your allies, which will in turn get taxed at their tax rate. If you want to help one of them rush, then you could feed them right away--at the expense of your own expansion and economic development. Most feeders try to develop their economy a little bit first before feeding. The longer you wait to feed, the faster you can become bigger and wealthier. I mean, if feeding right away comes at the expense of being able to put up trade ports, is it really such a smart move? That depends on the situation.
Another thing the feeder needs to do is to distribute the feed to the right places. Imagine a 5v5 with four hungry mouths to feed, all yapping about how they need feed. If you could only feed one or two of them, which one or two should you feed? Should you feed the new player who had the misfortune of starting off in the "suicide" spot with two opponents on either side of him who is now screaming for feed, or should you feed the pro who is on the flanks? Which one will be a better investment for your team? Will the newer player piss all the money away by building ships that will just get destroyed by two enemy fleets or will he have the knowledge needed to deal with that situation (either dig in properly or make some colonizer caps and flee to the middle of the map)? Even if he is competent and knows what to do, is he still the best guy to feed? (He might make a good migration to the middle of the map and expand there and become healthy again with a new home world, but does that make him the right person to feed? He's not going to win the game for you, at least not in the next hour.)
6) Micromanaging. Again probably based on situation. When to and when not to? More for late game than early game? Anything would be helpful here...
Always...
7) I hate to ask this, but people seem worried the community wont last long. Myself and 3 buddies started playing online. Is it a bad idea to get the expansion now?
The concern is that people will lose interest in Sins when Starcarft-2 is released. I think it will have some effect but won't be a killer. The biggest problem is that Sins has always had low online player counts. However, if you log in from 2 pm to Midnight U.S. Eastern Standard Time you'll find other people and games, at least if you're patient.
I think that the player counts have actually increased in the past couple of months, perhaps because the price of the game has gone down. There are also a fair number of newer players around, too.
You and your friends don't need the expansions if you want to play online. Just start out playing together in Regular Sins and see if you like it. Besides, you have a greater chance of finding newer players in Regular Sins anyway. Do note however that most of the action is for Entrenchment, which is where you'll find the majority of the players, the pros, the regulars, and the larger 4v4 and 5v5 games. I don't think you guys will feel badly about spending $10 each on the expansion if you want to play online often. The starbases make the game more interesting and add additional strategic considerations. What I would wonder about right now is whether or not the community will accept and move on to Diplomacy, in which case you guys would need to shell out another $10.
Anyway, I think it's great that you guys want to come play the game online! 
Such a great game deserves to have more people playing it online. You'll find that it's much more challenging than playing against the AI but also more fulfilling, more suspenseful, and more fun. Try not to take losing seriously (even pros lose or feel embarassed often) and try to take something away from every game. Adopt the mindset that you don't have to beat your opponents to have fun but that you want to be as tough and as resilient and as big of a pain-in-the-ass as possible. The team game is very different from single player because if the team wins, you win. So even though you yourself might be getting beaten, if your teammates win their battles, your team could still win. Thus, sometimes it's not about whether or not you are going to get defeated and forced to migrate, rather it might be about how you get defeated--were you a huge pain-in-the-ass who occupied your opponents' attention so that they could not focus on developing an economy or focus on your allies--or were you easily steamrolled with all of your ships being destroyed and thus decisively knocked out of the game?
