If you go with the sponge filter - you should run the tank with both filters for about 4 weeks. Then remove the hang on the back. You should test for ammonia once every other day after you remove the hang on back for a week or so just to make sure no ammonia builds up.
I run several tanks with sponge filters or hanburg mitten filters. Mostly seems to work out.
I am switching from the hang on the back filter to a sponge filter and need to know if I can just turn of my hang on the back while the sponge filter is on because there is to much movement for my fish!
As stated above...put the media from the HOB filter in contact with the sponge for a week or two, and the sponge should become active. I would keep a good eye on water parameters during the transition, though.
To seed a new sponge filter I always rinse some nasty old media in the tank water so the sponge filter can filter it out and become seeded as it does. The water may be cloudy for a few hours or even a day or two but I've never had and issue doing it this way.