as long as they discuss it within game chat, it is not against the rules. the only sort of diplomacy that is against the rules is secret diplomacy. here is the rule to clarify it for you...
CC Rules wrote:Rule #2: No secret diplomacy
Any form of diplomatic discussion between opponents must be posted in the game chat in English or in a language that all opponents understand. Diplomacy includes, but is not limited to: proposing truces, negotiating alliances, and coordinating assaults. Secret diplomacy can be hard to prove, but if you suspect it you should leave the players in question appropriate ratings. If you feel certain about players engaging in secret diplomacy consider reporting them in the Cheating & Abuse Reports forum by posting a new topic.
i personally hate alliances, think they are for weak players, and rarely call for them. however, they are part of the game... good luck...-0