I reuse hair all the time. When I remove it, I take each plug, tie a knot in it and then snip it off the pony. I dip the knot in glue and let it dry. Then when I rehair, I use a pushpin to widen the hole and then string a bit of fishing line through to pull the plug through the hair hole, leaving the knot inside. Once the pony is completely rehaired, I use a paint brush and put glue along the hair inside to help hold it in place. Whenever I bait out a pony, if I'm keeping the hair, I automatically create the hair plugs. That way, when I decide I need that color, it's ready to go and doesn't take as long to finish up a custom. Though, keep in mind that doing this makes the hair a little bit shorter than it would normally be so a lot of times I'll reuse adult hair on a baby custom.
I have also been known to steal hair from Bratz/Disney/Barbie styling heads and other larger dolls, such as the 16 inch Strawberry Shortcake line. I've found that using regular size Barbie hair doesn't work as well. It's super fine and tends to shred when you pull it through the holes. Bratz styling heads have a pretty nice array of colors that the ponies themselves don't usually have, like shades browns, blondes and auburn. Styling heads are usually designed to take a lot of wear and tear from little girls so the quality is generally really good. I find mine at local thrift stores for a dollar, so the price is right, too. I recommend a thorough cleaning before use, though, because I've found that little girls like to use makeup and chalk to try and "color" the hair.
As for the mohawk hair style, I think if you use thick plugs it will help the hair to stand up better in the holes. Then just trim down to the desired length, I guess. I haven't actually tried to give a pony a mohawk so I don't know how well that will actually work. Maybe someone else has better tips. Good luck!
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