I don't have any experience about this but I can share my thoughts.
Stick to two types of metal, it should be easiest considering different melting points. If you use more the metal with the lowest melting point should(most logical) be between the other two(or more) types of metals, so that the metals of higher melting point don't touches because then they may not stick and if you should get that high in temp the "lower temp" metal will melt and may run away and ruin it all.
Materials with clean flat surfaces fuses together easier under pressure so tightening them good and even should work best I think. And I've heard that with a "very" flat and clean surface and enough pressure you can fuse metals together in room temperature. So with better prepared pieces the lower temp you need.
About the clamps to compress the billet I think you could use normal steel because the only touch the outer surface that you would grind of anyway so from that point of view it doesn't matter. If you have stainless it will warp and be more sensitive to heat and may crack in the cooling and things like that, and if you also have stainless bolts they most likely will get stuck when heated and you will have to cut them off.
To work the material you can forge it with an hammer and anvil or/and use a roller or wire drawing plate.