In the staff window you assign staff and as you said, there are 107 assigned.
In the Design window you can assign those 107 designers to work on different parts, likely you only assigned 88 there and 19 are not doing anything. So if you want to use those 19 to do other things than designing, you will need to un-assign these in the staff window.
As you're pretty much building your team up during a first season, I would do that afterwards going into a second season. So guess that's about lvl5-ish by that time.
What to look for is experience, preferably at 20 as a designer gives it's best at that level.