Hello and welcome to the world of betta breeding! It's a time consuming and heartbreaking one, but one I enjoy immensely. Below I will answer your questions to the best of my ability based on show standards, textbook stuff, and experience
1. Though two different tail types CAN breed together, it creates an often unsightly looking fish and will not be accepted as a show quality fish. Combining a crowntail and a half moon will result in a "combtail" which is essentially a half moon with a few short and stubby rays on the edges of the tail. It's always safer to breed bettas of the same tail type as that's where the gene combos are well known and documented (with the exception of double tails, NEVER breed two double tails as it can cause stunting and deformity)
2. In my opinion, purchase a completely new pair. One from a breeder of known lines. I'm not sure where you got your bettas from, but unless you 100% know their genotype and the genotype of their parents (grandparents preferable as well) then you should not breed that fish. By breeding fish of unknown background, you risk passing on disease and cancers and overall weaken the betta species in general. Aquabid will sell you pairs (shoot for siblings) that are ready to breed, or even proven breeders. This is the best way to go, but a local breeder works just as well for this purpose. Perhaps it's just me, but a few other fellow betta breeders report that dumbos tend to be harder to breed. Technically it's all down to each individual fish, but for the sake of success, try to avoid dumbos.
3. Your male is definitely too old to breed. It would be a shot in the dark that he'd actually care to reproduce and raise his fry, but even if he did, the fry wouldn't be likely to have a high success rate (which is alarming given the bettas already low success rate). Ideally, you should shoot for a 7-8 month old male to breed with your female, who should be about the same age as well. Another issue with breeding old bettas other than physical and fertility issues, is the issue of ethics. Old males and females are less likely to recover from a rough spawning and this can severely injure or even kill them in the end.
4. There is no set success rate, it depends on which style you choose to use, hobbyist vs. Thai, your set up, the quality of your breeding pair, the quality of your live food, your schedule, and hundreds of other tiny things we cannot control. As a beginner, you'd be extremely lucky to even get the bettas to spawn. You'd be even more lucky to get around 25 fry. Many say bettas are the easiest egg laying fish to breed, I disagree. I can tell you that successfully raising a spawn is no easy feat.
Now I know you've likely done your research and don't want to hear me drone on as I am about to in this bottom paragraph, but as a betta breeder and fanatic, I like to make sure that everyone goes into betta breeding responsibly and ethically. Have you thought of all the money that goes into this? You'll need hundreds of extra tanks, bins, buckets or whatever else you can house a fish in just in case you wind up with a large batch. Do you have the money and resources to deal with live food? Bettas and most fry require a diet of live food and live food is often hard to culture and is very pricey. Do you have heaters and filters for your bettas? Have you thought about what will happen to all your bettas once they've gotten to selling age? Have you arranged buyers or had a pet store agree to take them? If not, are you willing to house hundreds if not thousands of fish in proper and permanent housing if need be? Have you studied up on betta genetics? It's so crucial you understand how betta coloring and genetic patterns work if you ever wish to have success in the hobby. Have you spoken to an experienced breeder near you? Or even better gotten one to agree to mentor you? This last one may seem like a no-brainer, but have you done lots and lots of research via books, journals, films, and the internet? I don't wish to discourage you from breeding, I just want you to be aware the breeding bettas is a huge commitment and takes time and money! I hope you do end up breeding bettas and if so, I wish you the very best of luck!