Could I have say, a giant gear rotate on a ship using particles? That seems a bit too much but I honestly don't know
YES : http://www.love-from-russia.be/g4_swarm-6.25_million_poly.avi
two ring and one neutron star rotating each around a different axis... being part of a huge starbase...
No idea how to make it by using particle forge... i directly make it with notepad...
You can control a lot of parameter ... by example, you can have a ship who have the shape of a Anvil... a secondary mesh, looking like a hammer, using the ship fx called via particle... in the particle file, you can control the angular speed, the min/max rotation angle, etc.... so, you can make a model with a hammer hitting the anvil in a repeted way... it is possible to chain particle effect and so create by example a robot arm animation...
Only one thing, weapon and ability hardpoint don't work on a moving section... flair work, by example a blinking light...
Location of the hardpont who call the particule effect and the location in "space" of the called secondary mesh is very important...
For the time being, i am working on a other well know starbase with a rotating section :

Model will be over the sins limit of 50k poly... these limit is in fact a xsiconvert.exe limit, not a engine limit... the short video at the top of these post ( with 125 starbase ) show 6.5 million poly onscreen ( using a nv 9600 gs 256 mb )... so, the engine is really strong... for rare object like starbase or maybe expensive capitalship, you can go over the 50k poly without problem... the trick is to split your model in different mesh who don't use more that 50k poly... by example, a main hull with less that 50k poly, a secondary mesh with detail less that 50k poly, a other mesh for all weapons less that 50k poly, etc...
The real problem is the sins mesh limit... if you make a small model with several animated part, you will have a bunch of small .mesh files... and sins have a very low limit for his total amount of .mesh ( i think that it is about 800 for all the game )... let say that you made a new model for replace one of the planet ( gravity wheel model ), where you have a central sun, with 5 planet who rotate around the sun, where the second planet have 1 moon who rotate around it, where the 4th planet have 3 moon who rotate around it... well, if each planet/moon use a different texture, you will need 10 .mesh for create these star system in a single gravity wheel...
A other problem for ship and starbase... during the construction phase, element called via particle effect are no visible... they only appear onscreen when the construction is done...
In short, a lot of thing is possible with sins... the engine have a huge potential... sometime, work around is needed but a lot of thing can be made... by example, it is not difficult to transform star in fur ball by editing the sun fx... with the coming of rebellion and shader 3.x, more wil be possible... only problem is that you need to know shader language and that ironclad have never release any info about the name of variable who can be used ( miss info about the internal of the iron engine )...
The only thing who is really heavy on graphic level is the collision detection... not a problem in case of a main object with plenty of moving piece since collision detection don't apply on section of the model generated by particle effect...