My experience for buying manila seedling trees and growing my own from Ataulfo seed is the following.
30% will grow rapidly and make excellent rootstock for grafting. (It is best to let them grow in the ground 2 years prior to grafting)
50% will grow OK but at a reduced rate relative to the 1st group. These are the majority and will be adequate for grafting in the future.
20% suck and have poor vigor. Some will die or grow slowly. These make poor samples for grafting.
It takes 1-2 years to find out what you have. It is hit and miss. Personally, I grow Ataulfo seedings in the spring and separate the top 30% for grafting after about one year of growth. The others I discard.
Any mango seedling tree has the potential to be a great rootstock but it takes time to find out what you have. This is a hit and miss. If you buy Laverne Manila Rootstock at home depot it the luck of the draw. I have several mango trees that are doing great and several more doing poorly. One died recently in it's pot. Remember these are seedlings and they are all slightly different and will respond based on the soil type and environmental conditions of your microclimate.
You need to buy 5 mango seeding trees and keep the top two. It's a numbers game. Also, buy seedling trees from different sources to increase diversity and increase your chances of finding a winner. Look at the lower rootstock when you buy. The larger the girth the better in general.
Good luck to all.
Johnny