Well with a sump you actually want LESS flow in theory because it gives more surface time for the bacteria and algae to utilize the resources. Just get a maxijet 500 or something and the gph
will still be pretty good. You don't really need to blow the hillstreams away, just a moderate current will work.
If you think about it though, and under tank sump uses a pump to pump water up anyways so the mechanics are the same.
Some people put a powerhead on the input and output so in essence you could do the same thing and the over tank will do the same as the under because the under will have gravity assistance in flowing into the sump while the other has gravity assistance flowing out of the sump. The water displacement would be the about the same.
Okay about the seeding... If you have green algae in any of your other tanks, remove that decoration or gravel or razor blade it off or something, then put it in the algae grow tub. I used to collect it from the stream but upon further inspection there were all sorts of nasty stuff in there. XD
So just try to find a friend who has green algae growing on a rock in his tank or something.
I think they eat diatoms (brown algae) as well so if you have brown algae, you could do that too.
Keep me updated on the Hillstreams!
Best of luck in whatever you decide, - Raymond