I don't believe it's possible to "over filter", however, it is possible to go overboard. EG: 2 Cascade 1200's on a 55 gl tank would be overkill.
Having said that, in my opinion, one 1200 would be just right. I use 2 Rena XP 3's on my 175 gl.
All brands of filters are notorious for over-rating the applicable tank size of their filters.
If you feel the flow rate is too high, you could put on a spray bar or throttle the flow down with an in-line valve.
Enjoy! They are a good filter. I especially like the valves supplied.