well as a rule for sizing, prob should say no as peter said, but it depens more on the individual fish tempermant.
I would say try it and by buying them all young and watching them grow. There will be minor territorial disputes but if anything gets too out of a hand, one or more of them may need to be removed so be ready if this is needed.
Yer well my intention is to get them all as small as possible so I can watch them grow I have read that this is my best chance for them to all get along
i envy you. trying to introduce new tankmates with my oscar in his 180 gallon tank has proven to be more than problematic. raising them young together would work great with those fish. skip the firemouths though. pick your oscar that is small enough that it is hiding behind the larger ones at the lfs. an early respect for other fish will most likely form their personality for the better. build up the tank with driftwood and coarse gravel or sand but leave lots of swimming room. cycle the tank first.
So the firemouths would be no good is there any other cichlids there size that I could put in this tank as I really want to have a few smaller fish swimming round as well
I thought that green terrors and jack dempseys grew just as big as the oscar..
Well the tank comes with a wet dry trickle filter, I was going to use that until the fish got to a decent size then add a large canister like a fluval FX5 or hopefully the smallest Eheim 3 electronic i think the model is 2076..
Jd's max at roughly 8" for a full grown male... I believe GT's are the same while maybe a little bigger (~10"). O's can reach 10-12" for a female and on some occassion upwards of 15" for a male. so they are actually 2/3 to 3/4 the size of an oscar.
Cons are usually a good choice, but unless you want them to take over the tank, only get one as a breeding pair will hurass anything in the tank... no matter how big. They are pretty good at holding their own against larger fish.
So even if the firemouths were put in the tank when the oscar and severum were the same size as the firemouth there would still be an issue with them being terrorized and eaten lol..
I really was hoping that I could have a pair of firmouths in the tank as well I really like the look of them