Southwest Paddler is a great resource for river flows: http://southwestpaddler.com/indexTX.html
Typically, between their "Minimum Flow" and the bottom number of their "Optimum Flow" is a good flow rate for anglers.
The Brazos below PK I did a 3 day float to Rochelles around 65 CFS; had to do quite a bit of portaging. Did the South Llano on a 2 day trip from Second Crossing to the city Park at 45 CFS, did quite a bit of dragging. Did the Llano River from City Park to Yates on a two day float at 110 cfs, and did minor portaging. These trips were loaded down with camping gear, so there would be less dragging for a typical day trip.
Did a day float down the Frio around 43 cfs and it was OK, had to do a a couple portages. The Medina and Sabinal (A real gem of a river and probably my favorite) I've only wade fished. The Sabinal is not a big cfs river, but the is very much kayakable; I've just never done it.
Here's a post I made to help plan river trips: http://www.texaskayakfisherman.com/forum/viewtopic.php?f=1&t=251189