a 30 gallon is fine for 4 kuhlis provided they are the only bottom feeders. make sure you get a sponge or some other way to block the intake for the filter.
I have noticed that sometimes on this forum people don't read posts carefully enough before replying to them (perhaps so they can jump in to give advice?). I would just ignore this talk of 4 goldfish...
I think it's also important to remember to take the advice given on this forum with a grain of salt, and use it to supplement your own knowledge and common sense. I have learned a great deal from this forum, but if I blindly followed all the (sometimes contradicting) advice, I'd probably be in a tail spin.