1) What's the youngest Bettas can breed? 3 months
2) Should I pick out a female that is smaller than the male or around the same size? Either one.. just not bigger
3) Do they have to be the same type? Veil/Veil, Crown/Crown? Or can I mix them like Veil/Crown? They don't have to be but your going to want to research tail type genetics. Veil tail crosses tend to be very unsightly and it's extremely hard to place fry from such crosses.
4) Is there a good chance they'll be interested in each other or do I just have to put them together and hope he builds a nest? You have to condition your fish to breed and then introduce them properly. 99% of bettas will spawn if things are done correctly.
5) How long do the fry take to hatch and how long until their colors start showing? the fry take 24-48 hours to hatch, another 24-48 hours to become free swimming and usually begin showing color at around 4 weeks, sometimes earlier, sometimes later depending on the genetics of the fish.
If you look at the "Breeding Questions" Sticky you can find some helpful links to help you begin your research. I would also suggest you peruse some of the breeding threads in this section.