This is a very big problem that you cannot use several levels of soldiers in the game, for example, level 4 and level 5 soldiers can be used in one attack or more. This is possible in most games like this one. I think this is a weakness now and I hope it will be fixed in the next update
While you cannot put soldiers of different levels in the same squad, you can put them in the same march by having each special ant lead a different level.
This part of game play is difficult in early levels, when you are rapidly unlocking new levels of soldier ants, but you will outgrow this phase quickly.
In the early phases of the game, it is easiest to race to level 6 ants, and then begin hatching rapidly. This helps to avoid this annoyance.