It does not work the way you're saying.
It's either this or that.
Not really.. You can set up certain plugins / configurations to run only on certain maps so I firmly believe you should be able to set up both modes on the server (albeit not at the same time obviously). Then again, I doubt ZE is any popular and even then there is no need for a dedicated Zombie Escape plugin to run ZE maps.
Anyway it would be pretty messy to run two types of Zombie plugin on a server, some plugins would bug and would have to be disabled depending on whether it's an escape map or not, overall too much work for not very much though but as I said before, you could just use the current ZP plugin and add ZE maps, I don't see the issue with that (?)
PS: Zombie escape really isn't that fun, so even though it is possible, I would say no to it. I don't think people would like it.