A Kenyi, an Mbuna, and a Pleco walk (swim?) into a tank...

My existing 20L has a striped bumblebee Mbuna, a striped blue Kenyi,
and a small Pleco. The Kenyi is somewhat younger, and therefore
smaller, than the Mbuna. I am cycling a new 55Gal tank right now
(currently being helped along by a few 60 cent "zebras"), which will
be their new home shortly. When that time comes, I will be adding some
more fish to the tank. However, the yellow Mbuna is extremely
aggressive. He gets seems to enjoy chasing the smaller Kenyi around on
occasion, as well as terrorizing, but not hurting, the pleco. This has
been like this for a couple months, so there's no harm coming out of
it. He has his territory defined on one side of the tank, and the
Kenyi has his on the other.
Recently, the brown algae in the tank has been growing exponentially,
more than the pleco can handle. So, I thought I'd throw in a CAE.
Well, that was a good idea until it became lunch for the Mbuna about
an hour later. All that to say: What other varieties of fish can I add
to the new tank once it's established? I'm primarily worried about mr.
Mbuna eating/attacking anything remotely smaller. I guess I should
stick to fish the same size or larger eh?