check out the bulk reef supply dual reactor, I have a phosban 150 and 550 and two of the BRS duals and the BRS reactor is my favorite, quite a bargain, very complete and easy to setup. Phosban reactors are better for hanging on the side of tanks/sumps though.
Technically GFO has a tendency to need to be run run at a lower rate, as Krish and others said you really don't want it to tumble. I run the two BRS reactors (4 chambers) off one maxijet 900, the first two are full of GFO and the last two are carbon. I have the included ball valve on there but I run it wide open.
I have in the past but wouldn't recommend stacking GFO and carbon in the same reactor, its just a pain to clean/change and keep them separate. I read the study where they show the effective life of carbon is really about 3-4 days and typically exhausted after a week. GFO typically lasts months. I change out the GFO when phosphates start creeping up (normally it lasts around 4-6 months) and the carbon I change one canister out every two weeks or so. I leave it on the system longer than it says is useful but I don't rely on the carbon for filtration and use it more as a supplemental thing.
Prior to using reactors I would lay bags across a bubble trap in a sump to try and maximize the water passing through it. On my nano I would leave a bag in a HOB power filter.