Yup, I've got another chapter 59 question.

I sent a discovery request by certified mail to the def in TDCJ. It was signed for by a guard (I presume, because it's not the defendant's name).

Does that count as service for the timing of the summary judgment?

TRCP 21a does not require that the party sign for service of process...only that the pleadings are to be delivered to the party. Signature by a guard at TDCJ-ID does not invalidate service. If your service is challenged, you might want to get an affidavit from TDCJ describing how they handle certified mail. In short, I would go with the date the guard signed for the documents and deal with any challenge from the inmate if it comes up. Your alternative is to make arrangements with TDCJ to get the inmate's personal signature. In critical cases, we have arranged for the local sheriff to go out to the unit and personally serve the documents on the inmate.