For heating the tank I would recommend you to use multiple heaters. That will help keep the tanks temperature constant throughout. Otherwise, one part of the tank would be considerably colder. I would suggest 3 of the fully submersible adjustable temperature heaters like the Aqueon Pro 200 or 250 instead of the ones that are constant temperature in case you have to raise the temp for fighting ich or if you have a warmer (or cooler) temperature need for the fish. The Pro 200 is rated for up to 75 gallons (X3 would be 225 gallons) while the Pro 250 is rated for up to 90 gallons (270 total) which would not overwork them in a 200 gallon tank.
The fx5 is rated for up to 400 gallons. Yes, it seems to be a lot, but more capacity is better than having something close to the size of the tank. This would give you basically 3 full filterings of water every hour. Depending on what is going into the tank you may need the extra capacity, especially if you are putting fish that create a lot of waste (like goldfish) into the tank.
Let us know how it works out!
Edit: What are the dimensions of the tank?