The number of ant troops that you can assign in a formation slot depends on the special ant.
Considering the tremendous pace of growth your colony will experience early on, it will likely become a bit confusing as what to prioritize next as several tasks are completed on new opportunities for growth and expansion become available.
But development-type special ants are not good in combat, but these development-type special are good in gathering resources outside the nest or provide special bonuses when you station them.