I wouldn't recommend putting leave-in product in a pony's hair. Over time there might be a chemical reaction that could potentially ruin the hair.
Tinsel is just
special, no matter what generation. My technique to tame it is as follows:
1. Gently wash pony's hair with conditioner. You can comb it through at this point too - use a metal pet comb for best results, but don't comb too hard and try not to pull the tinsel.
2. Pat the hair dry with a paper towel so it's still wet but not soaking. Don't use a hairdryer as heat damages pony hair and you don't want the hair to be dry anyway.
3. Carefully wrap the pony's mane around its neck and wrap the tail around a back leg. The choice of back leg doesn't really matter, but in more complicated poses you can just wrap it around whatever leg's easiest for you.
4. Cut some long strips of
plain white paper towel. Never use coloured or patterned paper towel as the dye can sometimes leach into the pony! (Been there, done that, there's a G1 Italian Windy out there with a new yellow spot.)
5. Take the strips and wrap them gently around the mane and tail. They should hold in place by themselves if the hair is still wet, but you can use a tiny amount of sellotape (stick it to the paper towel, not the pony) or a hair tie to hold the strips in place in need be.
6. Put the pony somewhere safe and leave it to dry overnight.
7. If the paper towel strips are dry then gently remove them. If not then wait a little longer.
8. Voila! You should have a nice neat hairstyle with much tamer tinsel. Now place the pony somewhere where it won't get knocked over, or the tinsel will come free from its moorings and you might have to start again.
I've used this method on tons of ponies with tinsel and as long as the tinsel is in good condition it should work. Keep in mind though that tinsel is evil and mastering it is an arcane art.