Bout to start my first crop. Do all water pumps have "backflow" ability? Meaning what ever doesnt go down the overflow tube will drain back down the pump tubing, Or do I need to get some type of drain fitting? Also any one got a rule of thumb as to what size pump to get for any size of system? I will only be growing about 3 at a time.

Unless you install a check valve in the line they will all do so otherwise the water couldn't flow out of the tank up through the line. A general rule of thump is 100gph for every ten gallons of res needed, ie 40 gallon res 400gph pump. 3 at a time is a small flood and drain. If you are going to get the meters and etc. necessary for it, you might consider dwc. No separate reservoir, takes less height out of the grow area. Filth Fletch has a post how to build one, The 30 Minute Aero Build, Might want to take a look at that as well VV