[tophat665]

The crabs need some way to get out of the water from tme to time. If you can lay hands on a well seasoned piece of driftwood (or don't mind taking the time to season it yourself - boil 20 minutes, dump the water, repeat until the water isn't colored) then you could lower the water level in the tank enough that the crabs could climb up on the driftwood and be out of the water.

I don't know as I would keep any rainbows other than possibly threadfins in a 10 gallon. They generally want more swimming room than that will give them.

 

[tophat665]

look into some of the dwarf brackish puffers

Dwarf Puffers are freshwater. There are some smaller brackish puffers - Figure 8s stay fairly small, methinks, and green spotted, but they are too big for a 10 gallon. The thing with puffers is that they're fairly smart, and need a large amount of fairly stimulating space to do their best. And that's on top of being territorial and predatory and messy.

Insofar as I have been able to determine, the only brackish species that works in a 10 gallon for full life cycle is Bumblebee Gobies.