If it was me, I would get the rock, then decide what you want to add and just research each fish to see if it going to...you know, eat all your shrimp/crabs/ etc. My priorities are definitely what is on the rock first, but I'm not really concerned if something is going to eat all the macro algae for example.
There is going to be a happy medium regardless what you put in unless you specifically choose stuff to decimate each type of organism. You may just have to replenish a hermit crew for example.