Horses be horses. Lots of good commentary here for you to sort through. I'm just going to toss in my experiences, xD I was raised on a horse farm. My mom raised Appaloosas, we stood two senior studs (Prince's Jim and L.A. Colt 45, a son of Colida,) and three junior stallions for when the old dudes kicked off. Every year we had a foal crop of 5 to 15 bebbes. Et Cetera, et cetera.
Geld when you want. There are pros and cons to early and late. But, a colt is usually fertile when a testicle reaches 2" (51mm) in diameter. Obviously there are outliers, so, use that measurement as an idea. Also, get ready to explain to the neighbors what in heck you're doing crawling around under there measuring your colt's jimmies, lol My mom would toss any ungelded colts into their on bachelor pasture when a testicle was about 1.5" across.
Colts are turds. They put their mouths on everything. They nibble, bite, flirt, chew, bite, gnaw, bite. Don't let them, it isn't cute, and can be downright dangerous. As my mom discovered with her first horse, a mutt TW x Saddlebred colt she got when she was 14. She spoiled him rotten, and Dad shot him when he was 11 because he nearly killed 5 year old me. (this was half a century ago in Central Florida; no one rescued jerk stallions.) Don't even let them think about it. Stallions flirt by nibbling a mare's neck and behind her knees and hocks. If he's nibbling your knees, he's a naughty boy.
Tie a rope round the mare's neck, put a strong halter on him, and run a rope from him to her 'necklace', just long enough that he can't get behind her. Unless you want a rodeo when the rope slides up under her tail, xD Then go riding. Let her take the brunt of his shenanigans. When he rears at you, swing her butt into him. Or walk her off. That rearing also isn't cute. Nothing like riding your mare and two forelegs come down on each side of you.
Weaning; there's as many opinions on it as there is on gelding. And mares also have their opinions. One of my own mares kicked her filly off the teat at three months. Done wif dis crap, take yo teef elsewhere! Well, her sister also had a filly the same age, and she swooped in and took on nursing both! Some mares love babies, some don't. Just like us. I let her nurse both until they were 7 months old. Finally I couldn't afford her feed bill any more! Such a cow!
Separating them was hard. The first filly was a brute and jump-plowed through every gate between her and her auntie nurse maid. Followed by the other filly daintily picking her way over the rubble. Fillies can also be turds, xD I ended up having to string two strands of hot tape on every fence and gate,... Kept them apart for two months. Worked for the adopted one, and sorta worked for the other one. Mamma made it to 28 before kicking off, her filly is now 17. And the joke was that Mamma would still lift a hind leg so her 17hh daughter could have a snack. Both were always model citizens. Nursing late didn't ruin anyone.
And finally, (thank goodness you say?) Horses are herd critters. Regardless of weaning or not, if you separate any two horses, there's going to be a lot of hollering like the world is ending. Horses are very dramatic. Extra, even. xD If you know someone with another foal, goat, donkey, llama, pony, gelding, camel, zebra, Great Pyrenees Mountain Dog you can borrow, toss the colt and the visitor into their own pen together. And ignore the theatrics. Reduce her grain for a week. That also helps.
Welcome to the adventure!