Just to set things straight, a Firemouth will outgrow that tank. I had a pair of FM's that got a nice 71/2 - 8 inches a piece.
The convicts wouldn't really be too big in that size tank, but with everything that you have in that tank, once everyone reaches their adult size you are going to run into problems with lack of territory and space for all those fish. As they get bigger, they will require more personal space and if there isn't enough they will be aggressive and harrass each other to the point of killing one fish or another.
The Severum needs a larger tank as well, but it seems that you are aware of that already.
The barbs will be fine in the 29.
Honestly, IMO if you have the money and space, upgrade to a larger tank like a 55gal if you don't want to part with any of those fish, otherwise look into finding homes for the severum and FM's at least.
With good feeding and clean water conditions/dilligent tank maintenance all those fish will reach their max size regardless of the fact that they're in a 29.
Good luck with everything and please keep us updated on how things go and what you decide :dance2: