As Clay said, the ferric oxalate (20% solution) and metal salt (10% silver nitrate solution for kallitype) can be mixed at room temperature.

I use sodium citrate at room temperature with kallitype. There is really no reason to bother to use it at a higher temperature, as you might want to do with palladium printing to get a warmer tone, because with kallitype the color and warmth of the imge comes from toning, not from development.

BTW, it takes ferric oxalate a long time to go into solution. After you mix it you might want to put the solution (in a tightly closed bottle of course), in a hot water bath at about 130 degree F and give the bottle a shake every fifteen minutes or so. If you just mix it at room temperture and leave it standing around it might take overnight or even longer for the FO to go into solution. Silver nitrate, on the other hand, goes into solution readily.