size category smaller than they usually fight" is
This is a underestimating, pikimins are much smaller them humans, small size type 2 is not one category smaller nowhere
I cannot envision you would make the argument coming from the other end- that combat experience means nothing compared to fighting something much larger than yourself. A trained marine, for example, would retain their combat knowledge from battling other humans when battling, say, Godzilla- it would need adaptation, but it is still valid combat experience.
This is the oposite situation you are putting there, imagine instead a mosquito fighring... A pig for exemple, the mosquito ofcourse can sting and suck the pig blood without much effort, but make the mosquito the size of the pig and see It just be smashed by the big body slaming It
Pikimins are used with flying things tring to catch, bite, sting, grab, smash, stiing them and have show to use oponents atacks to wait for oportunities to defeat them, like waiting It to atack, dodge and be trow over them from behind
The Stinger is AI controlled to kill shit with little other functionality. The fact that it happens to be battling enemies some centimeters smaller than it is traditionally used to is not some magic situation changer where they have every disadvantage- it is mitigating of their experience, not erasing.
They aren't made to fight directly trough, the own profile of It and was alread stated here to be a spy machine, It's made to do what the mosquito would do to a pig, It's only mean of atack is jabbing them selfs foward to sting It's oponent, It don't overpower them or try to outskill them, thouse things try to get close by not beingh noticed
What is much more dificult whem the thing you are fighting willenot have more dificult to spot you them a human has to spot drone flying around their head
And again, White pikimins can alone damage metal pipes enough to make them stop leaking poison gás, so if you try to bring the made of metal argument again, no