In that conference scene, you must remember, that Tessio was complaining about "Barzini chisleing in to my territory and we do nothing about it". It wasnt until Michael denied their request to recruit new men that Clemenza and Tessio asked for permission to form their own families. Remember the dialogue:

CLemenza: Godfather, you once said the day would come that Tessio and me could form our own family, until this day I would never have asked. I must ask your permission to start our own families.

Vito: Do you trust my judgement, then be a freind to Michael