The Really Right Stuff PC-PRO Round Panning Clamp with Screw-Knob Release (B&H# REPCPRO) features each a 3/8"-16 threaded socket and a 1/4"-20 threaded socket on the underside which may be used to mount it on top of many ball heads. Having said that, since the 3/8"-16 female thread is more centrally located, we'd prefer to use that specific female thread and to add an adapter such as this if you want to mount it on a male 1/4"-20 thread: Oben 3/8"-16 to 1/4"-20 Reducer Bushing (B&H #OB1039) https://tinyurl.com/5xpnkf48

The bottom of the Really Right Stuff PC-PRO Round Panning Clamp with Screw-Knob Release has threads so you can screw it into a monopod. And it also has a bottom Arca plate so you can use it on an Arca-Swiss clamp.

Yes they will work together.

While we're not aware of any tests where the PC-Pro Round Panning Clamp with Screw-Knob Release was used on the Peak Design Travel Tripod, they should work together easily.

I use it on a Manfrotto tripod. You can unscrew the ball head from the tripod and screw the panning clamp in place of the ball head, or better yet, you can replace the mounting plate on the pan head with a RRS quick release and clamp the panning clamp onto the pan head.