Ok, here is a link that has many of the Fish Mate filter setups:
http://www.pond-people.com/pond_filt...fish_mate.html
From the looks of it these are made for small ponds with very few fish. Just a few koi will overwhelm one of these things, IMHO.
It also looks like the connections to the plumbing are pretty small pipes. And it looks like these are pressurized.
So a 2000gph pump might be too much pump depending on which one of these filters you have. To get 2000gph with small pipe you'll need to get a little more power than with large pipe.
So I'd recommend the W. Lim Wave or Performance Pro Cascade 1/8hp pumps. There are a number of options with those pumps and they push a bit more pressure than the Sequence 750 line.
For a submersible I'd recommend an Alita.