Can I add cream cheese to crusting buttercream to cut the sweetness?  If so, will it still crust?

Yes, you can add it.  As for whether it crusts, it depends on your fat to sugar ratio. To cut sweetness in mine, I add a tsp or two of lemon juice. It balances it out without giving it a lemon taste.

Thanks so much!  I think I'll try the lemon juice.

Try adding about 1/8 tsp popcorn salt....It makes it have less of a sweet taste but you won't taste any salt  Been doing this 40 years