@sozobe,
I think you'll find you own comfort level somewhere between being more permissive than usual and feeling that they're taking advantage of you and your hospitality. Those limits will be pushed and, as with the triangle stuff, there may be one or two specific kids who push them harder than the rest. And, yes, if they're going to congregate at your house they're going to want to eat whatever they see. It's understandable that you didn't want to say no, but now that you've been the good guy it's ok to say, "No -- those are for sozlet's lunches this week. Here, have some of .... instead."
I like George's idea of designated stock for the gang. We have a snack shelf in the pantry that's open territory. It only gets stocked during major grocery shopping. If it's something that I don't want the kids to get into then I put it somewhere else.
It's going to take some time to get more comfortable with the gaggle, and some of that will fall to sozlet too. You want to support and embrace being the gathering spot, but that doesn't mean you have to come completely out of your comfort zone.