The shooters are a massive balance issue for several reasons, the two main ones being:
1) They do not have to get into physical contact with their target which allows big hordes of them to deal ridiculous combined damage as they are essentially unlimited in the amount of ants that can attack (unlike melee units which can physically block each other and prevent each other from applying damage to a target).
2) They usually are completely unaffected by any form of stun, slow, confusion or similar debuffs, allowing them to apply much more effective damage than the melee combat units.
The ability that allows them to MASSIVELY increase each others damage is just the icing on the cake.
I'd much prefer to see shooters in a more supportive role (applying debuffs like stun/slow/armor reduction/decrease regeneration/damage reduction, etc.) than as active damage dealers, especially as the Formica rufa group will be the ONLY ant species that has units with a significant range.
(Technically all formicine ants (Lasius, Formica, Camponotus, Nylanderia) have the ability to spray formic acid, they are just much more limited in their range (it's more like a short-ranged flamethrower). Even some myrmicine ants like Solenopsis invicta (red imported fire ants) can spray their toxins around like a flamethrower by waggling their gasters really hard.)