If the current is too strong, they have a hard time swimming. If you give them plenty of places to get out of the current they will be okay. I have one in my 55g and I have two filters on it. There are lots of places for him to get out of the current and he is doing just fine.
Yup, the long flowy fins make it hard for bettas to swim against the current for more than a short burst. I had one for a while in my 30 gallon, and he was doing fine for a while. Suffered some late night aggression that he didn't survive though. That's one of the other things, other than bettas themselves being very agressive towards anything that looks like another betta, any fish than has a tendency towards fin nipping won't resist beating up a betta.
If reduction of water flow isnt possible, hiding spots will do the trick. As stated, something to hide behind or within is a good idea. Are you sure the other fish in the tank will not nip at his fins?