if it's for a tournament, just post using the (Others) tag in Other Teams and specify the rules of said tournament at the top of your thread. Also you must make a choice between the two Pokemon when you post your team, if you're alternating just say it in the Pokemon's description.
you can only post complete teams. If your team is ou, you have to post in ou teams, if it's not, post in other teams with the appropriate tag. If your team doesn't have a specific tier you should probably learn more about competitive battling before posting