Kept rotkeils for probably 15 years, much of the time with groups of congo tetras in the tank, which are nice in their own right. Would expect any tetras of similar size to be fine with rotkeils (or most tank bred green or gold severums). Other complementary fish that could work in a 75 could include some smaller geos, if you wanted to go that way, a medium growing fancy pleco, though these may tend to stay out of sight much of the time (depending on tank decor), or you could add one or a few smaller cichlids (depending on their size and temperament), which could include angelfish, medium to smaller acara types, and others.
Filtration is a matter of personal preference and opinion, there's a number of ways to approach it and not one single right or wrong answer. But something to know is when it comes to beneficial bacteria, it doesn't matter how many or how big your filters are, the beneficial bacteria colony will only be whatever size is supported by your particular tank, in terms of its nutrients, bio-load (total of fish, how much you feed, etc.) and overall balance. You can have ten filters on the tank, it won't increase the beneficial bacteria beyond whatever is actually supported by your particular tank and fish.
...In other words, add as many filters as makes you happy, but you don't really need them if you know how to run and maintain the AC 110-- or a number of other filters approaching their capacity. Not saying this to argue, just something not everyone knows.