What kinds of pumps (gpm) would I need for a 20 plant SoG? I plan on using a drip system with drainage back into the resevoir. If it matters I'm gonna be using organic nutrients.

hell for a drip system an aquarium pump! not filter of 300 gph is more than enough to run 72 drippers at 2 litres an hour, so should be fine for your requirements
if you get a smaller [less powerfull] pump the head[ hieght in which the water can rise] is greatly reduced. i have oumps called 'project' they can operate properly when only in 1 inch of water! and push a head of 36 inches!,
doesnt mater what type of nutes you use!, but is advisable to fit either a spong filter to the pump, or install a filter to the resevoir. a UV filter would be even better, but not entirely necesary.
a pump or airstone for circulation is necesary! and possibly a water heater depending on your climate control. ideal water temp is 22 degrees centigrade! no higher, unless rooting clones when it should be 24.

good luck

welcome to the world of endless possibilities,,,,,hydroponics!

I'm going to be using this for 20 soil plants (sea of green clones during flowering) with very good drainage, Do you think it will still work? Or would it be easier to use the old watering can?