I'm a newbie gardener but one thing I have learned during the past year is that it doesn't matter how large a tree can get, what's important is your commitment to keep them on the desired size, and control that no tree outgrows the others.
That said, I have three avocados (Hass, Fuerte, Reed) spaced about 6-7ft. As they grow, they will eventually collide, and this is where I need to make a decision about how to deal with them. As of now, my idea is to let them merge into each other, or probably give more priority to Hass over the others.