Welcome to PCF!!! When I first joined, I thought it was very tacky to have what's called a mixed set, which is a different denomination from different casinos. I soon came to learn that this is actually one of the coolest things that you can do, because it really lets you choose the colors and edge spots that you want in your collection.
With that said, I feel that the reason that it works so well is because most people don't mix different molds. So if you're looking to mix chips from completely different manufacturers, I don't know if you would be happy with the result.
Edit: to be a bit more concrete in my answer above, consider the differences in the way that the edges of the chips feel, and even the height of the chips. You may end up with stacks that are different heights even with the same number of chips. It might feel weird to grab a one and a five because they feel completely different.