What people want when they're breeding is to do it in the most efficient way possible--meaning, breed pairs that yield the highest quantity of desirable fry vs. undesirable fry. This means that the most efficient breeding pairs are high quality individuals from reputable breeders who keep keep track of all of the pairings they've done, so you know what traits are in the parents, the parent's parents, and so on. This way you know what genotypic traits the fish are carrying--no surprises that could give you an undesirable spawn to waste your time and resources.
You want to use fish that complement each other and achieve what you're going for. For instance, if you find a male you really like, but his anal fin is too long, find a female with a short anal fin, preferably related to the male, and you'll have more fry that look like him, but with a chance of getting shorter, more balanced anal fins.
Basically, you don't want to breed willy-nilly. You should have some knowledge of what traits are dominant and recessive so that you don't cover the fish's desirable recessive genes with dominant undesirable traits that are much harder to breed out. You should also be aware of show standards, so that the fish you breed will be desirable to the mainstream. Generally speaking, mixing tail types doesn't usually have very good results, with some exceptions. For instance, the veiltail (longfin) trait is dominant over every other tail type, except double tail and crowntail (web reduction), because these are co-dominant--mixing will just give you messy veils. Doubletails should not be bred to other doubletails due to an increased occurance of deformed fry.
When breeding... is it bad form to breed different fin types to each other? Or certain colour combinations?
well, i'm not a breeder and i don't know much about betta gtenetics but i do know that ct to anything else is generally bad. vt is usually no bred to anything else either. any combination of delta, super delta, halfmoon, or over half moon is generally ok.
If you want to start breeding seriously, to make your own line or something, I suggest you start with solid colors to your liking and of the same tail type. Getting good genetics from a reputable breeder is a good idea.
Once you have more experience, then you can try crossing them (both tail type and color).
I would not suggest CT for beginners because it's more difficult to maintain their good finnage (in terms of show quality). Then there's the HM which isn't too difficult if you have a good starter. But can be a bit tricky if you use a mixed genetics. The easiest, IMO, is the PK. Their short fins aren't difficult to maintain or improve. Besides they're divided into 3 classes; traditional, asymmetrical, and symmetrical. So either way you would have show quality.
Which mutation is valued more? .... IMO, it's personal preference. Every mutation has it's plus and minuses. Just keep in mind that shows value balanced and symmetrical fins and color.
I have wondered though--on occasion, I do see veil tails that are a very deep brown color, probably some form of red, and instead of blue iridescence, they have hints of a fuchsia-purple iridescent color. If someone could create a dragon line out of this color pattern, it would create very nice, shiny purples.
there's a magenta purple vt at my petco!!! and i just saw some females with light bodies and purple fins and petsmart! i bet they'd make some pretty babies, but idk anything about breeding. jus sayin :P