Sounds like something a nurseryman trying to sell grafted trees would say.
I may also have a wider range of what I consider an acceptable avocado than some people. I like most Mexican-race and Guatemalan-race avos as long as they don't have fiber issues in the flesh or ripen too unevenly, which are the main things that could produce loser seedlings. Some are a bit bland, some are more nutty tasting, some have slightly vegetal or earthy flavors, but as long as it's eaten when properly ripe, I've never had any with flavors I found particularly offensive. Oil content should be satisfactory in seedlings of Hass or Stewart. West Indies race avocados (i.e., FL/Caribbean types) can sometimes have more unpleasant/funky tastes, or overly watery flesh, but that's not relevant for you.
The number of years to maturity is the huge unknown. Your 7' trees might be a couple years from flowering, or on the other extreme they could be 20 years from flowering. Grafted trees should start producing within a couple years or so.