I use a small grained rounded sand in my tanks. Yes, the turtles ingest it all the time but it passes through the gut harmlessly. But I know what you mean about larger gravel.
I have used Fluval internal filters and have found them to be nearly useless. Even with only one small turtle they become clogged within a few days. Good for moving the water around and that is about it.
Will look into the H.O.T. Magnum too.