It is probably no necessary for cross rails, but if you are super worried about him then it may be a good idea to get some
Even if I'm only doing an 18" cross rail?
If you are worried about it, just thow a pair on. It is not a huge deal not to use boots (I saw a World Cup horse last year bare legged) but I prefer to. For jumping I would recommend the open front boots for the front legs and the fetlock boots of the hind. Hope this helps!
I may be paranoid but I never ride without boots! Jumping is something you should especially look into supportive boots for. Tendon boots are solid enough to protect the leg, they're lightweight, and they provide support. Plus, they look snazzy and are easy to put on. In my experience, opinion, and based on some things I've read, polos shouldn't be your first choice for either support or protection. They're a bit time-consuming to put on, and also tend to become hot, which may damage the tendons. However, I've read some contradictory things about horse's legs becoming hot. Ah, the reliability of the internet.
One of the most important boots to have for me is bell boots/overreach boots! Good ones, preferably no-turn ballistic. They're usually around 20-30 USD. One of the clearest things I remember is riding my mare out in a flat pasture, encountering a small dip, and hearing her back hoof scrape the ballistic of the bell boot. Gave me the heebie-jeebies and I've religiously booted up my horses since.
I always use boots when jumping even if they aren't high, just gives you that extra safety comfort.
I personally would always jump in brushing boots, it'll protect them regardless:)
Boots are good for safety and they will definitely help support your horses legs as he jumps. So I would suggest buying boots, or at least polo wraps to start with 😊