Males get to 4.5-5" with females being smaller. The theory behind overstocking is to spread aggression throughout the tank. It is better to have a larger shoal than one where a pair or two could gang up on a single fish. Doing it correctly means a large, well filtered tank with plenty of "caves" to establish territory.
Return the the odd fish to the store if you can and keep the pair in the 30. You could add other fish but if you let them have the 30, you will definitely have some breeding and the parents will raise the fry. It is educational and fun to watch but I seriously doubt any young child will understand the impact of their actions. You will also need a plan for the future of the spawn. I have my convicts in a 6' 100 with Oscars and silver dollars. The SD will not harm them but the Oscars eat them when the parents quit guarding them. Any that I save are sent to lfs or people on this site that want them. Once they breed it won't take long to be overrun with them! The first brood or two might be small but subsequent broods will increase in size. Sort of like "practice makes perfect". My last brood was about 75 fry. Survival rate was no less than 50.
You could use the 30 for them as it would be ideal. That would leave the 75 for other fish. JD would be ideal to feed off the unwanted fry to, but that is out for legal reasons. You can use the 75 for your showpiece tank like you want. You have the option of filling it with almost any fish you can legally own. They are literally a pefect size for most fish kept by those in the hobby. It goes without saying that it is too small for tankbusters, which still leaves you a ton to choose from.
