Should I just merge em?

While Canas and Garon are still limited to 20, I still cant recommend foddering off a 5* over a 4*. I say merge them.

I've merged my 3 Micaiahs and haven't regretted it. I would 100% fodder Garon over her.
If you're going to fodder Katarina, maybe consider Swift Sparrow?

I'd merge the Micaiahs, since DD has limited usefulness in an A slot and is available elsewhere. However, Katarina is completely outclassed as far as red mages go, so I'd leave her for fodder.