I would be careful, dont get Apistos from the same complex (there are 6 of them) eg Macmasteri and Viejita are the same complex and can hybridise if they breed. I would stick to the ABC (Agassizzi, Borelli and Cacatouides) of Apistos if you want a few different ones in the same tank. But with a 30G (approx 120L) tank I would only keep 4.
<rant>I hate measuring tanks in gallons as there it varies between US and UK measurements...litres are universal, 10x10x10cm of water is a litre. No wonder so many NASA things have gone splat due to incorrect conversions!</rant>