nice page that

the general rules for modules (as i've figured them) are:
- downlinks have exclusive slots on top of base (game auto zooms in that area when you try to build one) and there's only space for 3 per base.
- hangars are like downlinks, but their slots are the big rectangular parts in the middle of the base.
- all the other modules can be built on the non-exclusive slots all over the base.
- weapon modules have additional exclusive slots where you can put them. iirc, they're over / under the hangar slots and on the edges of the base (top / bottom).
- special modules such as shield generators, sensors, etc, can only be built once per base (just like for crafts).
- weapon modules have a limit, probably so you don't go overkill with weapon modules

i think it's 10 or something, and it applies to beam and missile modules separately. in other words, you can have up to 10 beam and missile weapon modules in the same base.
if you want i can give you screenies of the specific slots for each module type