The Prime will detoxify ammonia and nitrite, so it won't hurt your fish, but it will still be available for the cycle to complete and to get back to normal. That's really all you NEED. Bio Spira is the good live bacteria, and it wouldn't hurt.
I had a cascade 1000 that I got for free. I wouldn't use just that to filter a 120g with pbass. You should probably put the other two back on it anyway.