I haven't fished with the TXL, but I own a couple of SPLs in 2 and 3wt. I also have an Orvis 1wt and a few 2wts and a few 3wts, so I have some ideas about ultralight rods. I should say ultralite lines.
I know that your question was specific to the TXL, but really the line (taper and weight) determines what you can cast (how heavy, fluffy, far, quiet, into what kind of wind etc.). A 00 and a 1wt are really best suited for fishing very small flys in windless conditions short distances where a quiet presentation is most important. I think that they are also a novelty and that has some fishing value, but a two weight, and even a three would probably do as well or better under all circumstances. I don't think that a 00wt line would be very good for casting a #12 bead head.
While the 00 might be the "lightest" rod (or line rating), a fiber glass three weight might be every bit as sporty as the 00wt, and cast the heavier fly better too.
Having said that, I don't seee why you couldn't fish for pan fish with a 00wt. rod.
I hope that others will chime in.
One cast can change your day...maybe your life.