The "rule of thumb" is just that, a rule of thumb. Read in the articles section concerning how many fishes and I hope that helps. However, if you dont want to get too technical, the usual rule of thumb actually is 1" per 1G.
About time between adding fish. Well, that also depends if you used fishless cycling or not. If not, I would say around 4 weeks and only 4-5 fish at a time.