Neat idea, not sure it is really practical, but it sounds like a fun project.
As far as the weapons have you thought about the .50 cal gatlings, they make a 3 barrel and a six barrel version, falls right between the 7.62mm and the 20mm, more ammo carried than the 20mm but alot more punch than the 7.62mm, you have a tight space so some compromise is called for.
For the heavy punch the Soviets made a clip fed breach loading 82mm mortar called the Vasilik, it looks more like a small howitzer than a mortar and has a 4 round magazine, its been available since the 80's. I Wouldn't think it would take much work to convert it to 81mm and maybe go with a 10 round magazine or even a hopper. I think an 81mm mortar make enough boom to keep most people happy (unless its falling on you) and it also has many ammo options in case just big explosions are not enough for you.
Just a thought, also for your gun barrels you probably want to think about brass or aluminum tubing, it is much thinner than styrene and will look better. Its not much harder than styrene to work with except you need to use CA instead of plastic glue.