No rat-like race appears in any "Letters," "Forum," "D-Mail," or "Questions of the Month (Forum)" column appearing in Dragon #153 (January, 1990) through #266 (December, 1999).
During the last seven months I have slowly chipped away and re-read (and read for the first time, in some cases) all Letters and Forum columns--later renamed D-Mail and Questions of the Month, respectively--from the 1990s.
I read much about how great the new AD&D2e core books were. And about how awful the new AD&D2e core books were. And about how Dragon was giving too much coverage to AD&D2e, not enough to AD&D.
I read about how hard it is to get women into gaming, and how it really isn't. And about how hard it is to deal with teenaged players, and how hard it is to be a teenaged player when old folks treat you like garbage.
I read about Sally Jesse Raphael, and about trying to find other players in schools, and about getting your parents to play so they see there's nothing to freak out about.
I read about how poorly fighters are treated relative to paladins and rangers, and how illusionists really shouldn't have any special treatment, and why the monk should get between 7 and 10 unarmed attacks to every one weapon attack a fighter makes.
Should humans be allowed to multiclass? Should XP bonuses and penalties exist?
Why isn't Oerth/Krynn/Athas/Sigil/anythingbutToril getting more coverage?
How should castles be designed in a world with dragons but without cannons?
If (and we're really just saying "if", here, folks) there were a third edition, what rules would you keep and which would you ditch?
I tell you all this so that you can know this testimony is true: this year I have read all ~1000 pp. of reader letters to Dragon during the '90s.
Your rat-race isn't in there. Maybe a full-on feature, maybe an "ecology" article, but it wasn't in a letter.