In fact, virtually, you have no limit...
If you make a 500k poly model, simply save it in block of 50k poly... the trick is to use particle... you make the first block in .mesh, adding 9 hardpoint for anchor the other block... other block ( .mesh too ) are called via particule effect and use the normal ship.fx shader...be sure to increase the max amount of particule in your usersetting.ini ...
Only problem is during construction in shipyard, only the first section will be visible... particle effect will kick in only after the launch...
The real limit is mainly the hardware... but sins can show a huge amount of poly on screen... i remember battle between 3 fleet having 100 or more ship on each side... this is a lot of poly onscreen !!!
Method can be used in mamy various way... by example, for the lower left model show in the main post ( http://www.love-from-russia.be/DSC.jpg )... can make it very interesting in sins... save/convert the back in a.mesh, the clamp in open postion in b1.mesh and closed position in b2.mesh, the front in c.mesh... a.mesh will have two hardpoint for b type and c type, along one hardpoint for a colony effect...
Showed normally in game, a + particle b1 + particle c ... once start colony, show a + particle b2 + some ignition particule effect, particule c is killed and used particule c1 who show the front part moving toward the planet ( like for bombardment weapons )... once planet is colonized, kill everything related to the colonyship and create a new a part who who act like a defense station ( back section with 6 torpedo tube, a heavy long nuclear range missile, and two 8 barrels flak gun )...
Well, it was my idea when i have make these model but since XSI hate me, all my idea are in sleeping mode until somebody able to use XSI find it interesting... hmm, not really fully my idea but i have take it from http://www.youtube.com/watch?v=6Ck6Hcg2cjk ... model is made from these video too... can make it in sins look like in the end of the video, section separate, forward section land for colonize, and end section remain in space for orbital defense...
Any way, the point is the only real limit is your hardware, that trick allow you to use more that 50k poly for a model... at modeling level, it simply mean that you need to work in a modular way... and more important, thinking outside the box can lead to very interesting result... anyway, you need to know what trick will be used before begin to model...
A other example of thinking outside the box... people wish/ask dev for turret who track target... well, in some case, it can be possible to make it like in the pic below :

How? Simply model the ship without turret, but with the turret door... with the weapons hardpoint on them... create a firing particle effect who include the turret... in combat mode, turret will appear on the door, firing at target... since the turret is part of the firing particule effect, the turret will always be targeting at the ennemy... no more turret orriented on the left, firing at almost 90 degree to the front...
The trick with multiple mesh, particle, and fx can be used for a lot of thing... the B5 mod have use it first for make the rotating section from the Omegaship... but it can be recycled for make moving texture, light emiting section, stealth ship ( almost transparent, sins support it via his shader FX ), etc...