I would suggest watching the whole series first, and then the movies. The Beginning is fun and you could watch it after the first season, if you wanted to, but IMO it doesn't really add a lot that would make it required viewing before you watched s2. (There are some neat things it includes that aren't in the series, and I think it makes it a little more obvious that Kotetsu's not in a great headspace when the series starts, but it's not enough that you need to watch it before finishing the series.)