If you are still early in the game then don't stress too much about building up mother base. I don't want to say too much because eventually you will reach a point that will make managing your base very tedious if you have a large capacity of soldiers. You will have to review them and assign them each individually, for specific reasons that I won't get into due to spoilers. I had over 300 at the time and it took me well over an hour of micro managing every single one of them, not a lot of fun.
Another reason not to stress is because aquiring higher skilled soldiers is very similar to the way that raising your light in Destiny in was when TTK released. Basically, most soldiers that you encounter in the beginning will have E or D grades for their skills. As your overall average grade of soldiers increases, so will the grades of the random soldiers that you encounter.
In other words, the more D level solders that you find will cause the game to slowly start presenting you with C's in the field. Grab enough of them and you'll start finding B's. So on and so forth. Doing special side missions in which you are tasked with extracting prisoners or highly skilled soldiers will help boost you up much faster as their grades tend to be higher than the average soldier you would be encountering at any given time.
