I have small "pasturettes" (small pastures) and it does ok for my two geldings enough that during spring I have to use a muzzle for turnout. I have two smallish areas for grazing and two drylots attached to one that I can separate as needed to get my boys off the pasture. My second fenced in area can be for grazing and for training. My guys stay fat on air and I must watch any grazing and even with my pastures being smaller there is plenty of room for running and for some reason I have no problem with growing too much grass and still have to mow but I think that is because I use switching and my lots have time to get a break. I provide very fine plain grass hay year round and a pelleted supplement and I dry lot my guys at night year round for peace of mind and ease of checking on them after dark.
Until the grass is eaten down you have to watch for founder as many minis will over eat if not used to grazing even when eating just a small area down quickly.
I prefer geldings myself. Geldings are great to start with in my opinion.
Two is a great number. When I had three it was too much for me and none of them got enough attention... when I only had one he was very sad and lost his "spark" in his eyes, it was easier financially and I thought he wouldn't mind since I had goats, but I was surprised how much he missed our other boy. After 5 months with all being miserable including me, I got my other boy back and things are back to normal. . My two seems to be my magic number for me, I can brush them with a brush in each hand and lead them together on walks.
It is amazing how much poop two can generate as I learned this winter when I cannot get my manure out to the field and I have poo piled everywhere, so poop management is very important to consider. I scoop poops in my small pasture and remove as much as I can. My two boys fill a large wheel barrow every day. I use shavings and pelleted shavings for bedding. I am lucky we have a large farm and I can easily take the manure out to the field for removal (except for this beastly winter). If you just leave them to the 1/4 acre all by itself it wont take long for it to be either dirt or mud or both with poop mixed in if you don't give the paddock a break like I do with rotation and diligent clean up.
You will need some sort of shelter too.
It would be a great idea if you can find somebody locally with minis to mentor you. I had horse experience but not mini experience but was lucky to find a breeder that mentored me through the whole miniature horse transformation and she has become a great friend too over the last few years.
Minis are a bit different than biggies for quite a few things, but easier for new folks that have horse experience to step into the wonderful world of minis. best wishes.