Mark -
The weather for the Crystal City area during the month of March will probably be in the mid-50's at night, to the low to mid 80's during the day . . . but, it's just like everywhere else; extremes of temperature are not uncommon. Although Crystal is close to the border, it is still somewhat cool during the winter months.
Should the weather be warm prior to your arrival, you can certainly expect the Rattlers to be coming out of hibernation, but if you keep your eyes open, and be aware of their likely presence, you shouldn't have any problems. Scorpions are a far bigger concern than snakes! Be sure to check your shoes/boots before you put them on, and look carefully at anything you pickup off the ground. Areas of Zavala County are great for Arrowheads!
There are areas of Zavala County where the ground is littered with softball to football size rocks that make the footing somewhat difficult, so I would recommend a good boot for that little bit of extra ankle support just in case you end up in one of these areas.
This country is great for hogs and javelina, you shouldn't have any problem finding ample shooting opportunity. There's also plenty of rabbits (both cottontail and jack), badger, coyote, bobcats, quail (seasonal), and cactus, lots and lots of cactus. Be sure to bring along tweezers and duct tape for removing thorns, everything has thorns.
Hunting around feeders and/or water is the most common technique with the first thirty minutes and last thirty minutes of daylight commonly being the most productive, but they can be moving at any time of the day or night.
You didn't say if you were hunting high-fence or free range, but you shouldn't have any problem with shot opportunities regardless of which way you're going, there's just lots of hogs in this area.
There's also a great Casino in Eagle Pass ran by and located on land owned by the Kickapoo Indians. It's a first class facility equal to anything in Louisiana and they'll take your money just as fast. Deserves a visit if you have the time.
Have fun and enjoy your hunt, best of luck.
POP