What sealant should I use between a bronze or stainless skin fitting and a TruDesign ball valve ?

The same options as you would use for sealing any TruDesign composite fitting to a TruDesign composite Ball Valve.
TruDesign recommends the following sealants:

Adhesive Thread Sealants:
- Sikaflex 291i
- Sikaflex 591
- 3M 5200
- 3M 5200FC (fast cure)
- Bostik 920

Normal Sealants:
- Loctite 5331
- Loctite 55 Pipe Sealing Cord
- 3M 4200
- PTFE (Teflon) Thread Tape