Acrylic and glass won't bond. Silicon doesn't hold acrylic very well. Why do you need this bonded?
Use two 1 inch, just in case. It is always better to overbuild than under. If one gets clogged and you only have one, then what? 1 inch are good size overflows for a 30 gallon. Remember not to drill too close to the top because the water level will be about 1/2 to 1 inch above your elbow. You may also consider a mag 5, or put a ball valve on the outflow side of the mag-7 in case you need to cut back the flow. You could also split the return and have it return in a couple different places around the tank. You probably won't need additional flow in the tank.
Tim Marvin
(512) 336-7258