Size wise, FX6 is more than enough for a 200g. FX6 are built for 400g, so you have enough filtration, but to be on the safe side, you can have one more means of filtration just in case this fails.
I would run 2 for sure, I have an fx5 on my 75 and 125 and it works great, I was going to run 2 on my 300 before I went with a sump. I am going to run both fx5s on my 125 once I shut down the 75