My tank came with a MAG24 on the return, The guy that sold me the setup had it with one 1" drain for many years an never had a problem. When I set it up I used the 1" for the return pump and the two 3/4" holes for the drain for a bit of safety from snails clogging the drain.
I belive the size will work for you but the best advice I can give is DO NOT GET A MAG24. It is VERY noisy, It was vibrating my house and filling the downstairs with a constant Hummmmmm. After running for a bit the seal gave and leacked every were; repair was not posible for many reason I don't want to get into. So I started to run it summerged WOW that thing runs hot and warmed my 300g tank fast, and being under water did not help the noise at all. A MAG12 is a good pump but a Mag24 is not. There are many better pumps that size both sumurged and exteranle that are in the same price reange and you will be much happyer with. I replaced mine with a Reeflow pump will never run a submerged pump externally again.