also don't forget to turn of Randomized Start Position

and yeah it is possible, do it all the time

If you're only playing against the AI or with a few friends then you can just make your own custom maps in Galaxy Forge as MyFist suggested.  It would actually be very easy to fix if you want to modify the starting positions in the standard maps.  You will notice that some Terrans on the edges have been designated as home planets, like "NewPlayer1-Home" with the players going from 0-9.  On the <Edit> pull down menu select <Players> and then for each player you can lock the player start to a certain team--0 or 1 (or whatever single digit number you want, I suppose).  The number you want to adjust is "TeamIndex".  You can use that to set the starting positions anyway you want.

Then save the file with a new name to your desktop.  Find your Sins Application Data folder and move the map into the "GalaxyForge" folder and you're good to go.  It will show up as a Galaxy Forge or Custom map in the map selection window.  You can email the file to your friends and tell them to put it in their Galaxy Forge folder.  Here is a link to information about how to find that folder (read the 2nd post in the thread).  http://TinyURL.com/Sins5