Hi Kim,
from what i have read most dairy cows are fine outside down to -20F, so long as they has a place to get out of the wind (if any) and have access to quality feed. (per the family cow book, which ive also read

)
cows have an internal temp of 101F, so as long as their rumen is full they can create enough heat to stay warm. It has been down to -18 here in CO this year and our jersey/brown swiss cross is doing just fine. she is due in 3 weeks.
I think the biggest thing is to make sure the teats arent getting to cold and getting frostbite. may not be a issue in east texas.
i am not much help on pasture question, as i am working to restore our pasture also. i think a diverse mix of grasses is the minimum to shoot for.
we are in the process of building a 16x16ft milking shed, but it wont be done before the calf is here.
hope this helps.
http://www.cloud9farms.com/ - Southern Colorado - Zone 5 (-19*f) - 5300ft elevation - 12in rainfall plus irrigation rights
Dairy cows, "hair" sheep, Kune Kune pigs, chickens, guineas and turkeys