If the "black tetra" are G. ternetzi I personally would avoid them as they can be somewhat nippy and Firemouths potentially grow rather long trailers which would be a shame to lose. On the Corydoras front when buying them look for medium to larger sizes to add to the aquarium, it's not all that uncommon for a cichlid to choke on one. Given that your Meeki is around three inches there is a choking hazard of adding very small Cories.
