Right, it's among friends, so it's not going to be a big deal either way.
But I still think that the hosts should just invite everyone to a pizza party and leave it at that, because no matter how it's worded, telling people to feel free to order other things will sound like they are apologizing and acknowledging that what they are providing isn't adequate, but they are not hospitable enough to bring it up to acceptable standards, so they're giving their guests a pass to supplement (which otherwise would be extremely rude).
If the OP does worry that the menu she is providing is not a good fit for this group, then she should rethink the menu.
But she didn't say that -- she just was concerned about whether the fact that this group is used to going Dutch treat and ordering on their own from a menu requires her to announce that they are free to do so at this, a hosted party.
In my opinion, it doesn't, any more than inviting a group that usually dresses casually to a black tie event requires the hosts to inform them "But if you want to wear jeans, please feel free." You get an invitation that doesn't suit your preferences or customs, you either accept and do it a different way this time, or you decline.
The guests haven't done it the other way 500 times, and I'm sure they have all attended hosted parties where the hosts chose the menu. It's not going to kill anyone to eat something other than what they would have chosen for themselves for one meal, or else eat earlier or wait to eat later. I agree with those who have said that worrying too much about whether they will be caught unawares this time is overthinking this.