I discussed some of this stuff with Demian a while back, and here's the low-down as I understand it.

Weapons slots and damage capacity are restricted for the sake of genre expectation and realism. There's no reason in the underlying probability game why you couldn't add as many weapon slots and fire actions as you like, nor pay for and add as many points of damage capacity as you like. This is the perfect place for you to house-rule it however you and your opponent see fit.

Combo weapons definitely go beyond 2 frames combined, I think that after 2 frames you have to pay a bonus +5 points per frame (the first two are at normal combo cost, all past two incur 5 additional points over and above their normal combo cost), this is to stop people from building multi-frame one-shot combo weapons which may be unbalancing and certainly would be a nightmare for book-keeping purposes. It's in the book, so check it there.