Yes, you can trade up or down as much as you want, as long as the numbers match up.

The restriction of thievery on Gorons is because they physically wouldn't have the dexterity to do things like pick someone's pockets or pick a lock. If someone sticks their huge stony fist into your pocket, you're going to notice. Sneaking for the same reason; Gorons are all huge incredibly huge and heavy, and probably can't move silently or subtly if they wanted to. If you wanted to rob by force, you wouldn't need thievery anyway; just knock out or kill the person and take their shit.