For starters your tank is nowhere near heavily stocked I had over 80 fish in a 540L tank including adults giraffes dolphins etc before my tank crash. The fronts when they grow will take up some space in the future so it may look full then.
I would change 20-25% a week or even twice a week if you can be bothered, you will probably find that you only need to do them every 2 weeks (really easy to measure nitrate levels and find out how fast waste is building up) but more is always better. I would have been doing them from day 1, or are you trying to say that you had been topping the water up instead of changing the water?
Cories are a no-go have seen them in cichlid tanks before most of them had half of their fins missing and eyes missing etc, I have had luck with bristlenoses and they seem to do well in african tanks they do get a bit of a hard time (but so does everyone else) but mine still breed etc in there so they were obviously happy. What are you looking to "clean" is there algae everywhere or uneaten food? You could add some synos to help clean uneaten food and for something different..