@Kro My Kraton had no shims behind each sun gear (the 2 larger gears that sit over pins at either end of the diff case) but had one behind each planetary gear (the 4 smaller gears that sit across the diff case). I seem to recall my rear diff only having 3 shims on the planetary gears, too. It now has 4 and still feels really smooth.
I bought a set of Mugen diff shims. The kit includes 6 large shims to go behind your sun gears and 12 smaller shims for your planetary gears (enough to do all 3 diffs). I added the ones behind the sun gears, removed all of the Arrma planetary shims then replaced them with the slightly thicker Mugen ones and they're now buttery smooth with no play to be worried about. Your diff is likely to need a different combination of shims, but give it a go.
If you're interested in doing the same thing, look for 'Mugen Seiki Differential Washer Set
MUGE0206.