Bulk Head and Emergency line Questions?

[scar_11]

I just drilled 2 holes in my tank one for the return one for the over flow Should I silicone the bulk heads in or not?

I was also wondering if I should put a 2nd overflow line as a emergency back up. Do alot of people do this?

3rd question is I am putting a internal overflow box in should I put a rubber seal between the overflow box and the tank or should I silicone it?