I think the shrimp are a better idea. You can get cherries for pretty cheap, and they breed really quickly. Just depends on what your lfs will give you for them. The invert selection where I live is horrible, so I can sell cherries pretty easily. Yellow shrimp are another great choice, I just got 12 of them from Chikorita and they are doing great. They are more expensive to get, but they will also get you a bigger payoff when you sell them. Tigers are nice too, but I don't think they would sell as fast as the yellows.
I just got a 29 gallon to breed orange dwarf crays and endlers. It seems orange dwarf crays are pretty much impossible to find, and I found a breeder that will sell me a pair of them for $70. Pretty expensive, but I hope to get my money back after my first batch of babies
