Too many fish for a 20 gallon tank?

[shayna]

I have my tank set up, just letting it settle for a week before I get the fish.
I was thinking of getting 2 red-tailed sharks, 3-4 zebra danios, 1 plecostomus.

Also, I was thinking about getting kuhli loaches but heard that they need sand? I have larger river rocks in my tank. Would they survive in that?

Thanks!