canister filter and built in overflow

I dont think that would really work. First off canisters arent really what people use doing reef tanks. There are just to many issues with them. They are a pain to clean, which you have to do often if you want good parameters in your tank. Now if there is no way you can set up a sump they can be used but you will have to take it apart and clean it weekly. I did this for three years on a corner tank and its a big pain in the ass. Also for the cost of a canister you can set up a sump/refugium and that works light years better. You dont have to buy a premade sump you can pick up a used small tank of go by petco and get a 20 long on the $1 a gallon sale and convert it your self. As far as a return pump I got mine at aquatraders and it works really well cost me all of 30 bucks. So for my 45 I bought a premade sump was $120 and the pump for 30 hoses cost around $25 So I got a really good system for less the $200 my last canister cost me around the same and was a pain in the butt. In addition a canister filter works on a straight siphon. Not really how you would plumb that to a drilled tank. also canisters when clean push around 400 gph (depending on size) my return pump is rated at 712gph. Just my :twocents: but you will be so much happier with a sump/refugium and will have better water quality.
if I don't know the gph of my built in overflow how do I know what size of return pump to get. And also can I use a 10 gal tank as the sump?
What is the size of the drain line? Typically a 1 inch drain line is 600gph a 1 1/2 is around 900
Although canisters can filter water well in emergency situations they end up becoming detritus traps after only a few days and cleaning them will get old fast.