Good advice on any large fruit as mentioned above, if the tree grows fast and large, it may have a large root system. But I have been growing white sapote in my small yard for over 8-10 years now, mainly seedling trees I grew from seed. They do have a large root area, much more than a loquat or mango or citrus tree of similar age. I have many citrus trees in ground that are over 30 years old, and they do not have the large size root area as the white sapote. But mulberry trees are the ones with the bad invasive root system that will grow outwards from the main trunk, maybe over 10 feet under soil easily. I'm eventually getting rid of my large trees one at a time, putting them back in pots.
White sapote trunks can get up to (10"-20") but most trees I have seen have good tap roots and the roots seem to stay closer to the trunk and not grow out as much as a mulberry. When I dug up my 20 foot seedling tree (10 yr old, 10" diameter trunk with 2 seedlings fused together), I only needed to dig about 6 feet in diameter of the trunk to get all the roots dug up, most were deep roots downwards, not going out. My other white sapote trees grow in similar fashion, don't see any roots going outside of 5-10 feet from the main trunk of the tree.
To answer your second question, don't trim off the main trunk/central leader branch until after you get your first fruits. If you cut it down, it may take longer to get the first fruits on the tree. I see this on my seedling loquat trees, I never cut it back until it has its first fruits, then I can shape or keep it to the desired height. My white sapote seedlings were the same, I let it grow as tall as it wants until I see the first flowers and fruits, then I start trimming it down to the final height I want to keep it. My seedling white sapote trees all fruited in 5-10 years by letting the central leader/trunk grow without being topped off.
