I dont know if anyone is right. With the right filtration and maintainence, you can do a 10" cat. I have a 72 bowfront so i alittle more than 2x a 30g. I have a 11" niger cat, 4" stripped raphael, 4" irwini, 2x 2" spotted raphael, 3" flagtail porthole, 12" severum, 6" severum, 3" frontosa, 2x 4" bichir, 4" royal pleco, 4" BN pleco, 2" gold nugget, 4" SAL and a 6" clown loach. all are fed M-W-F and with 50% weekly water changes, pH is 6.8, 0-amm, 0-nitrite and traces of nitrate. (i know some city water sucks but memphis water comes straight from the aquafer without nitrate so yes, my params are correct).
so if you cut my stock in half, it still excedes a 10" cat in a 30 g by far without any trouble. maintainence is key and frequent small feeding vs fewer larger feedings will help reduce changed of overloading beneficial bacteria and resulting in amm and nitrite spikes.