I have a miniature horse and live in the subburbs of Houston as well, so I can easily help you with that....but honestly I'm not sure that getting a mini as a companion for a full sized horse is a great idea. If your horse gets upset and kicks at the mini, it could easily be killed. We had a mini gelding killed by one of our quieter horses that just got fed up with him one day. Fatal kick to the head.
If you do get a mini though, you're going to have to feed it hay, not pasture. Miniature horses can not have much grass (more than 1 hour a day or so, if that) because they can founder or become obese, and you'd have to buy a grazing muzzle for it to wear all of the time.
My mini mare (34" tall, ~220 lbs, 5 years old) is normally used very often for driving and is in moderate work, so she'd normally eat more than your mini would if it was just a companion. Right now she is on stall rest (sprained fetlock joint) so she's eating close to what yours would eat. She is dry lotted normally and has no access to grass ever, unless I hand graze her for a few minutes. She eats 2 flakes of a small bale of Jiggs Costal hay (40 lb, so each flake weights about 4 lbs) daily, and half of a flake of alfalfa hay when I can buy it and it doesn't look like crap. If I can't find good alfalfa I sub in 1 1/2 lbs of alfalfa/timothy hay pellets.
The jiggs costal hay costs $11-14 per bale depending on the week, and I go through about 8 per month. It is included in my board but if I paid for it I would be spending $112 tops on the hay. I buy the alfalfa pellets myself, which are $12.99 per 40 lbs but I doubt your mini would need the alfalfa. She was on grain, (nutrena) which costed me $21 per bag and lasted for what seemed like forever (a good month, I'd say) but you shouldn't need that. Depending on the quality of your hay you MIGHT need a suppliment or ration balancer. I've used Enrich 32, just 2 cups, daily before, and it lasted me, again, about 5 weeks. That was in place of grain.
Then ofcourse, farrier work. I have a VERY hard time finding good farriers in our area (especially for a mini) and end up paying $55+ for a half way decent job, but maybe there are better ones in Conroe. Then you'll need wormer of course, and vet checks. Other than that you should be good to go.
A word of warning though. I've had difficulty finding good quality hay around her lately, so always break a bale up after you pay for it if you can, before you leave the supplier. Check for mold, weeds, thick stalks, burrs, etc, and demand a refund for the rest of that one is nasty. I've had so many of my bales LOOK nice but on the inside be horrible, that I just thought I'd warn you. And if it doesn't smell good, it probably isnt ;)
Also, for the full sized horse, I'm feeding a very malnourished, high maitenence TB filly right now and have her on a high-cal, high-fat, high PRICE diet, and we still about $250 per month on her feed including unlimited hay. She has ad-lib Jiggs Costal hay, 5 lbs alfalfa pellets, (WILL HAVE, still integrating it into her diet) 2-3 lbs beet pulp, 4 lbs Nutrena Mare and Foal, and DuMor Ultrashine suppliment. Her farrier work is another story though...lol
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