Yes, one slot per 5 tons.
You lose -1 Thrust for each 5 or fraction after you add. So you'd be down 4 to 1/2 or 2/3 Thrust with 3 ASMs, but must still pay the full cost to move at safe/max which will burn through your fuel quite quickly.
The internal bay is a quirk thus has no bearing on construction rules beside its limited wording, and is an optional rule that costs 3 Quirk Points which must be supplemented elsewhere (like to say take improved targeting). You can have both external and internal bombs. The external ones cost you thrust. The internal ones have a drop limit and risk catastrophic damage if your fighter is struck by 'ground fire' (not sure how that interacts in space) and the enemy rolls a 10+.
