I just requested to join the Mandos but the havent replied...I was wondering if they have rules against joining other factions because I kinda feel like spawning some time soon and id rather not do it on Polis Massa.

Factions are led and run by real people. They may have processes that must happen for you to join, or someone just may not have come online yet that can approve your application.

The single best piece of advice for this game is: be patient.

Regarding Mandalore having rules against people joining other factions; ask them, you won't find any faction specific answers in here.

If a faction is not replying as fast as you would like, it might be an indicator that the faction is also not as active as you'd like, or that they are too flooded with applicants to reply in a timely manner. In either case, you are always free to choose a different faction, though anything under a day or even two is a reasonable wait time.