In a crusting BC recipe that calls for heavy cream, can I use milk instead? I still need it to crust and I don't feel like running to the store.

If it's a small amount of cream used just to thin the BC, milk should be fine. You may need to reduce the amount you use though. Just add enough to get the consistency you need.

It calls for 5 Tbsp. Too much?

I actually use milk more often than not in my buttercream. I always keep some whole milk around just for baking puposes anyways. Slap my hand, I usually do the Rachel Ray thing and "just eyeball it" too. Just add a little splash, mix, splash, mix until it is the consistency that I want it to be. I'm terrible about following recipies exactly.