Hp can't say to many variables.
But the psi question really is an opinion.
See some will say 8 - 10
While others run 15 and call it safe.

The tune is so important to the question what's a safe psi?
We tell our customers daily driving 8-10psi
As long as you have good fuel and good tune.

 

What are the advantages and disadvantages on running a twin and a single?

 

The advantage to twins is that the spool time will be faster since each turbo is smaller. But it can be more expensive since you have two turbos to buy.

 

will gt3076's be enough to get me at least to 600with a good tune or the gt35's would be better?