It's hard to know for sure what you are dealing with so I will come at them as I would with a bunch I have purchased to grow out, style and resale at some point down the line. Without seeing any trunk details it is impossible to assume you have one or more trees that really can be developed as a bonsai without having to go through what I am going to describe.
I am not a big fan of putting Mugos in the ground to grow them out. Most of the time they go in the ground and stay there. You right now have bushy little trees,--- if you put them in the ground you will have bushy larger trees If you don't have some sort of idea what you want to do, or have to do, to develop the trunk this process is a wast of time and in the end,--- probably a wast of material. The most important thing in a Mugo, or any other Pine bonsai for that matter, is the trunk. The trunk of your tree will define the quality and nature of the finished tree.
The problem with putting them in the ground without making trunk oriented decisions is today's defects, solvable with minor pruning, will become major problems latter down the road that could smothered out more desirable, even if weaker, branches and trunk directions that could have been developed.
I so often hear people complain about the landscape nature of the Mugo Pine, it's just a bush that is round and all kinds of trunks and--- bushy, turn around and suggest the tree be placed in the ground to develop, putting the tree in the very situation that created the problems they complain about.
The Mugo is a mountain tree. It is adaptable to a host of environments and I have found them in landscapes as far South as Texas. However they need to be treated differently than JBP which is the standard two-needle pine bench mark for bonsai culture, wrongly so in my opinion, but it is what it is. You can only treat a Mugo like a JBP so long before it fails. I have been growing this tree seriously since 1972. I am constantly trying to learn more about the tree and one thing I discovered this summer is that the Mugo is quick to form a Krumholz in order to survive the environment. This is also why single trunked Mugos are not that commonly encountered in the Nusrsey trade. The Krumholz formations are more encouraged for landscape purposes. Single trunks have to be developed out of the beginnings of the krumholz formations.
This is a Mugo Krumholz located in the parking lot of the Hovenweep Museum in Southern Utah. This tree and three or four others demonstrate a couple of things about Mugos. This particular tree has been here for 25years according to curator of the Museum along with three or four others:
This tree was planted at the same time with the same group of trees and is about 12' high.
Sorry for the rant but this brings us to the point already alluded to; defining the nature and shape of the trunk. Until you discover what you have going on I would suggest that you place all of these trees in Pond baskets or colanders. These will allow the trees to grow but give you the ability to work on them at the same time.