That's why we use flat face flanges and elastomeric gaskets for FRP / Steel joints - they are easier to make leak tight. Although I will give odds that a system with more than a dozen or so such flanged joints will leak when first filled with water for hydro (before being pressurized).

Consult your FRP vendor for advice, and have him supply a field support engineer for construction and leak tesking the FRP.

The bending moment and axial force on the steel side of the joint will be exactly the same in magnitude, but opposite in sign, of the bending moment and axial force on the FRP side. Of course, the steel will be considerably stronger. We don't specify FRP for its strength or durability, we specify it for its low cost, light weight, low electrical conductivity, and corrosion resistance.