If you would like a pair of CA's then this is what I would suggest but you will definately need to keep a close eye on them and keep a divider or a hospital tank standing by because you will need to seperate:
1 pair of any on this list:
Convict
Rainbow
Sajica
Cutteri
Firemouth
or any other Crypto's or Thorichthy's.
I think you could do this with no problems. I have bred Convicts in a 20 gal L. Although I will admit it would be better to have a bigger tank but yet I think it will work just fine as long as you monitor and do water changes. Good luck!
Personally, I wouldn't consider Thorichthys which can approach 7".
What! 6" maybe but definately not 7!!! :lol: I have never heard of a 7" Firemouth!