What you describe is already what we're doing, deciding when a new
races is better than one of our others. Logically speaking, in a sector
of space as massive as what we're talking about, you're going to have
literally *hundreds* of worlds, possibly thousands, that have sentient
life. (I forget now the specific number of such worlds advanced by the
Drake equation, but it's on the level of millions of such worlds in our
galaxy alone.) You'd also have subset worlds; a world controlled by the
Centauri, for instance, whose inhabitants are not themseles Centauri, but
some other race. (Or species, more accurately.)

The pak'ma'ra (the carrion eaters) weren't just introduced for that
one episode ("Legacies"), they have been seen before and since, starting
with the very first episode. Deathwalker and Varn ("Deathwalker" and
"Voice") were both the last of their kind, or close to it in the latter
case, so logically you couldn't have it be one of the previously seen
races. So that only leaves two or three that we've brought in in the
course of a season.

We knew that when all was said and done, we wanted a minimum of 20
or so races who would be available to us at all times, and now we've got
those. We've just finished a 35 page breakdown of each race's history,
culture, language, politics, reproductive mechanisms and other areas for
in-house distribution. Most alien stuff will come out of this group,
unless, again, there's a specific reason for bringing in a new one that
would compromise the story otherwise. (For instance, the Drazi play a
role in several episodes next season.)

We're not just throwing them in helter-skelter; we wanted to build up
a solid "repertory group" of diverse aliens, and now we've got that.

jms