What size holes? 2 Mag7's might be too much for 1 drain unless it is a big hole. you might want to make 2 drains and 1 return. You can always bring fittings up over the tank to get a return. You could even run some pvc
on the lip of the tank with multiple outlets. If I could change one thing on my 75 gal
with the internal overflow, iy would be another drain. I would like to handle more flow but my overflow can't handle any more.
I would use 3/4 for most parts. The locline comes in 3/4 and 1/2 at most places and 3/4 pvc
and or vinyl rubing and fittings are easy to come by. Locline can be shortened. Just make sure you know the hole size for the size of bulkhead you want and size your bulkheads based on the flow you want to handle. Keep in mind if you ever go with SPS
that you might want more flow than 2 mag7's. Don't be limited by your drain(s). You can put a small pump on a big drain but you can't put a big pump on a small drain.