The stock pump that comes with the Intex pools is not nearly enough to filter for life. Very short term only and is more to keep some flow in the pool, not actually filter. Also based on the fiting you probably couldn't rig a decent pump to that outlet on the pond. Most likely you'll need to keep that stock pump hooked up even if its not running to clog the intake hole that the pool has and then just route your new pump piping over the sides. I would think that 6000-10000gph is enough if you are only planning on keeping those two species. I would also recommend a dual pump configuration in case one fails that way you have at least some flow/filtration just in case of failure.