Depending on the sex of your firemouths you could do a group of 12 comfortably.
You won't know the sex of them until they mature though.
Ideally females will out number the males.
I don't think you need any more cichlids, thorichthys are very entertaining on their own in groups. They also make a good visual display once fully grown.
I would definitely not add a convict. They are far more aggressive than thorichthys and will cross breed with them.
A good sized shoal of swordtails or sailfin mollies would be biotope correct and make great tank mates.
You could also half the number of meeki and add a group of thorichthys pasionis.
The yellow colour contrasts well with the red meeki and they are also found together in the wild. You could also consider thorichthys helleri, my personal favourite. They to can be found in the wild with meeki and pasionis around the catazaja region of Mexico.
Thorichthys helleri
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Man, I'd love to get some of the other thorichithys types but they are normally impossible to get hold of. The only other common one is ellioti and they don't really do it for me. I was asking the LFS in my new neighbourhood about lack of American cichlids in his shop and he said there's no demand for them anymore which seems to be true. Once I've got the tank up and its cycling I will start a serious search and see what's available. I think the convicts are definately off the list now. As for the dithers, I've always liked the look of the swordtails in your photos but once again in australia you commonly only get orange ones or line bred colour types like "tuxedo swords" which won't suit what I'm trying to do. This is why I will go for guppies most likely. I know it's not full biotope correct but will still look more natural in my opinion.
I'd actually like for some of you guys to come to australia and check out some shops and see the limited stock and quality and high prices we have to put up with here. Still it's always exciting planning a new tank and cruising around checking out all the shops for inspiration.