Could plaquenil (hydroxychloroquine) for uctd make me susceptible to serratia marcescens and stenotrophomonas maltophilia in two sinus cultures?

Maybe but less likel: Plaquenil (hydroxychloroquine) is an immunomodulator and can certainly increase the chance for certain infections. I don't think it is a very significant amount of increase though. On the other hand, Stenotrophomonas tend to be seen in individuals that have had recurrent exposure to different antibiotics. I would guess you might have had recurrent sinusitis (or other infections) where you were treated with abx.