I personally love doubles but I don't have very much experience with it.
Despite Fox's frailty, playing defensively and using your ridiculous speed to move around the stage, get a couple hits in and disengage quickly is the most effective strategy I have found with Fox. My partner that I play doubles with has called Fox the "Doubles Assassin" because the combination of his (initial) dash, vertical speed and fall speed enables him to position himself to kill either enemy very effectively. This is further amplified by Fox's really impressive frame data (especially on his kill moves).
In terms of synergy, my friend plays Pac Man and WFT. I think Pac Man functions quite well with Fox by offering stage control/zoning and of course any hydrants launched can be reflected back by Fox as well as his fruits (Melon is the most fun to reflect because it offers huge stage control).
I think Fox's frailty is best complimented by a zoning character or another speedy character to help keep stage control. Fox is a pretty good doubles partner, but I don't think he compliments heavies that well or vice versa, but that is purely theory as I don't have any experience with them.