Ideally you would want a pressure pump with about 500 gph per injector minimum, but I dont know of any pressure based pumps that are submersible. If you cant get a pressure pump upping the flow per injector to 800-1000 gph will work too.
If the skimmer is also in the sump, I would say you could use a mag 12 or 18 or equivalent.
If its not in the sump and is on a shelf(has some head pressure to get to it) I would do a mag 18 or 24 depending on the elevation.
Check the pump curves for what you are looking at to make sure it can flow at least 500 gph at the height of the skimmer neck(not the injector)
You can also get by with much lower flows if you use an air pump to push air into the injectors as well.