For land, you only get so much as a new player and have to save up to purchase more land expansions. I went to go have a look see and it seems you purchased an extra large stall which holds 20 horses but took up 25 acres and I believe you start off with 25 acres and you look like you can own 22 horses as you are also given, with your account a lean to which can house two horses.

I think if your horse does not have a discipline set, then you cannot board with others. You have to have a discipline set regardless if you use the horse for careers or exploring paths only but you can just tick off the discipline path I believe.