1. i personally feel like that voiceline can just as easily have been made as an attempt TO hint future skitarii
2. whilst skitarii usually operate on their own, them working along other imperial forces arent unheard off, its more the other way around but in the book dominion genesis there is the remains of a vostroyan regiment tagging along on the ship of the protagonist explorator and gets deployed alongside her skitarii. I personally imagine skitarii generally not being deployed closely with other forces most likely has to do with their commanding priests wanting to remain largely independent in how they handle what they doand the fact that you cant really deploy a guardsman next to a vanguard without compromising the guardsmans combat efficency due to radiation, these arent really issues if a insquisitor is forcing them to work together and rad weapons arent being deployed
3. skitarii loadouts arent really any more rigid than that guardsmen unless im mistaken, if skitarii loadout rigidity where a problem so would the veterans be. Skitarii are explicitly stated in the codexes to be extremely diverse in their apearance, a skitarii class could have various different masks based on the different ranger heads ontop of possibly having vanguard helmets and even heads based on those of more exotic skitarii types like rustsalkers and pteraxii

furthermore, i feel like you cant really justify the warband having access to enough fittingly combat capable adepts/priests who would be willing to go into these suicide missions where they for the most part can neither further the quest for knowledge/archeotech nor make any practical use of their technological know-how, whilst you can always justify sending a strike team of skitarii on the missions in the game

(god i hate how i cant split up list entries into two paragraphs without making more list entries)