I would go with 7 1/2 or 8. You want to get the pellet count up in a .410. At grouse ranges you should have plenty of pellets and energy out of a 3" .410 for grouse.
I use 8's in my 20 early season and go to 6's late season after the leaves drop.
In my 20 I like to use AA sporting (I buy them by the case for Sporting clays) but any good clays load (AA,STS,Gun Club or Top Gun) will work.
The premium clays loads have harder shot than the cheap "promo" loads. You don't need a premium copper plated hunting load for grouse.

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