Is it permissible for two individuals to pray in congregation between the Adhan and Iqamah, or does that contravene the creed?

It is impermissible to establish a congregation in the mosque before the regular Imam arrives. Rather, it is obligatory to wait until the Imam comes out and the congregational prayer is established, because leading the prayer in the mosque before the regular Imam arrives is forbidden except with his permission. Furthermore, doing so is an usurpation of his right and an invitation to chaos.