If those indexes are like any other index used in the game, it is just for sorting the order of the items displayed. When there is a tie, the game chose one to go before the other, probably whichever was loaded first.
So if someone else chose the same index, the worst thing that could happen is your class being not in the place you wanted.
As your classes are in the combat category, maybe you could use some indexes that make it be placed near other combat classes. (you can use decimals, like 2.5 to put something between 2 and 3)