In my experience (and my gelding has a VERY long mane) DO NOT BRAID!
Braiding breaks hair, especially if the mane or tail is thin. Think about it, we braid our horse's mane while his head is up, but when he puts his head down to eat, the top of his neck stretches and becomes longer. The hair is bound in braids, and thus gets ripped out. As for the tail, every time a horse swishes a braided and/or bagged tail, hair breaks. Braiding tails up might work during the winter when there are no flies.
As long as the mane or tail doesn't drag on the ground, there's no need to braid. If the hair really is that long or unmanageable, put it in loose figure-8 knots
instead. But please, I'm begging you, save your horse's mane and tail and don't braid.
It's definately easier to grow hair when your horse has the hair gene, but it isn't necessary. Instead focus on his diet. Make sure your horse is getting his omegas! Give your horse a multivitamin (if he needs it) and adding a skin/coat supplement can do a lot for his hair. Not only has SmartShine Ultra made my horse super glossy, but his mane and tail have also exploded in growth. BioMane makes supplements specifically for hair growth, so maybe compare ingredients?
As for hair products, I can't recommend Mane-ly Long Hair
enough. To help your unicorn have the longest hair possible, I would get the whole line. The shampoo leaves hair more moisturized than before you washed it. Their detangler is a bit expensive, but I can guarantee you'll never go back to your old one. It's light, non-greasy, effective, and doesn't freeze in the winter (bonus!!) I never touch my gelding's mane or tail without some detangler in there. When I brush through his mane and tail, there isn't a single hair stuck in the brush, which means none is breaking or being pulled out. Never use whitening shampoos or detergents on your horse. Never use coat polish spray on his mane or tail, nothing will dry it out faster.
I sound like a total spokesperson for this company, but I wouldn't recommend it if I wasn't totally in love with the products.
Sorry for the long post, but I hope my experience helps you obtain your long mane dreams!