You don't have to but they would be happier and you'll likely see them out and about more often. I wouldn't worry about overcrowding. They're only 2"
Personally I'd use 3/8 but you should be ok with 1/4 so long as you don't bump it real hard. Also I'd suggest using weld-on 40, tilt and pour method
It depends on which species of ghost shrimp you have. If you have the ones with pinchers then they will likely kill the rcs.
As far as guppies go
okay... guess test kit is a must!! thanks for the advice ^^