as anna said bettas are very territorial. i managed to pick the softies up though =) they're really nice little fish, colorful and spunky. i keep one of mine in a 10 gallon community tank with some tetras and kuhliis, as well as a few invading snails. provided the fish aren't fin nippers (IE: tiger barbs) a betta should be fine with quite a few little fish, like tetras and danios. but the guppies are a no go, my LPS kept a betta in with a few monster female guppies and some little males and all the fish were chewed up in two days =( it pretty much wrecks the pretty fins you bought the fish for in the first place.
i've never bred bettas, but i think a female can only breed when she shows dark vertical bars (dark-bodied, i had a light female and never figured it out, and she was eaten =() and when the male has a bubble nest going.
i don't know new zealand currency, but here in canada you can buy a nice veiltail mutt betta for anywhere from 2 to 5 dollars. deltas, halfmoons, double-tails, crown-tails, lyre-tails, and pretty much any specialty-bred betta can go anywhere from 10 to a hundred and ninety dollars, i've actually seen a beautiful melano lace halfmoon for that much =O
females and males should NOT be kept together unless you really know what you're doing. i kept a single cambodian female with my boy Donahue and he chewed off her ventrals within two days =( my LPS has kept a trio of females with a male without any bad results, but i'm not sure if that's all that wise...
bettas can live 2-3 years, and grow to be about 2.5" to 3". if you buy a purebred betta i think they're bigger, seeing as they were kept because they were the best possible breeding stock, but pet store mutts are just that; the mutts.
=D i actually gave some wisdom! yaaay! though i am a bit new to this, bought my first betta about a month ago and my newest was yesterday. buuut i'm pretty sure most of this is accurate, i've plucked it from about ten zillion replies to posts i've made =)