76-78 sounds good, just try keeping it steady. being a smaller tank, with lights it may rise fast, which may pose an issue.
the canister filter would prove best run empty and let the live rock in your tank be your "filtration"
do yourself a favor and skip crabs then, atleast to begin with, they can always be something added down the line if the desire is there. i also suggest scarlet hermit crabs if thats the route you go there. i just dont like them because ive had them kill snails for their shells, but the scarlets seem to be the best behaved. i would also skip on getting a cucumber, this tank isnt large enough for one.
do you have fish considerations or not yet?