Yes, you can. You can also mix any other kind of plastic you want in with the baits with no ill effects - despite what the label says.

Can you mix Gulp?

Yup you can throw them all together, ive had all of mine in one jug for over a year and they worked great my last trip. Mixing gulp with other plastics would probably cause some issues though.

Does Gulp bait go bad?

As long as they stay moist, they're fine. They won't go bad or anything. As for the fishbites, you can still use them if they're dried out or not.