My apologies if this has been covered before. The Tractor Beam seems to be a cannon upgrade, but the Mist Hunter doesn't have a cannon slot. Does the Mist Hunter title allow the Tractor Beam without the slot? Meaning, you could have the Tractor Beam, but no other cannon?

Yes, that's correct. The card text telling you you have to equip it overrides the rule that normally you couldn't.

Remember the golden rule. Cards override the rules. You must do what the card says.