starting a fiddler crab aquarium, i have a few Q's

[zeig]

by the way!
can i keep an asian red clawed crab with a fiddler crab or will the red claw be too aggresive?

also, do i need a hydrometer?
what salinity should the water be for my crabs? i hear its healy disputed lol..

P.S. (one day later)- if its just gonna be 10 gal in a 20 gal tank do i need to cycle it for crabs? also, do i need a heater? its summer atm and my stick on thermometer reads 76-74ish but by the end of the day ive seen it at 80 (highest ive seen).

BUMP...