Re-doing plumbing Q's

[burrows14]

Just a couple quick questions that are probably easy answers but I am not sure of because I have never done this before. I am gonna build a new sump and want to completely re do my entire plumbing system. So my Q's are....

I have a 120 gallon tank with 2 overflows 1"/returns 3/4"

1) Can change out my bulkheads while the tank is filled?
2) Do you have to use potable pipe?
3) What glue is recomended?

Any other tips/suggestions would be greatly appreciated!

Cheers
Ray