Per the rules, you cannot create alts to get around an account being banned (so if one account is banned, they all get banned, technically). There isn't really a rule that you can't have alts on the forums, if you are behaving properly, per se.
I thought alts on the forums were banned as well, but if you read the rules posted by Horus, it only specifically calls out people who make alts to avoid their ban. It says nothing about alts in general. I will admit there is some grey area when it comes to having forum alts if none of them are banned, but the way I read the rule, it's not technically illegal to have alts in the forums as long as none of your accounts are banned. But maybe I'm reading the rule incorrectly.