be prepared to have a lot of them mixed up. once they run out of vertical climbing supports, they will flop about and create a rat's nest as they grow around each other. many types of hops produce the majority of their cones at the top part of the plant (i.e. within the rat's nest). untangling them might not be an option, you'll end up ripping the bines but not separating them.
growing centennial and cascade near each other is a good idea since they are relatively similar and can be used in the same recipes. maybe dedicate the trellis to "IPA hops" so anything you can't distinguish can go in your "mystery IPA hops" collection.
or, as you say, you could not worry about it and just have a giant mystery hop collection that could have anything from williamette to columbus
personally i would find that a tad limiting since you can't control what's going in your brew (i.e. aim for a style), then again it's kinda fun to not know how hoppy your wheat beer is gonna turn out.