I think the open tank gives a better chance of survival for the small ones as long as you can get food to them and so far, this seems to be working out most excellently. PE Mysis should be fine. Inspite of them being FW they are a staple in the Marine environment for corals and fish. I would look for alternates to change out a couple of times a week though. Before you leave, If you have access to some small hermits, leaving a couple in the tank (snails without an operculum will likely be as good) when you are gone. Even if he can't get them out, he can work on them and if really hungry will likely succeed.