I don't think anyone on this board keeps pipefish. The following would be
just my opinion from what I've read, for what that's worth.
Pipefish are very similar to horses and both should be kept in a tank that is designed with their needs in mind. Clowns would not be recommended tankmates for either pipes or horses. They are slow movers and slow feeders and clowns would eat most the food and also keep horses and pipes stressed out due to the fast paced activity of clowns.
Also, when buying horses, it is recommended to get them captive bred for many reasons. I don't think your pipefish would be captive bred, so if you do consider purchasing it(despite the compatability issues), find out what it is eating and make the lfs show you. Actuall see the fish eat or don't buy it as you may have trouble getting it to eat.
There are some great threads at the top of this forum about horses. I recommend google for better pipefish info. Hope this helps a little anyway.