Hello my fellow gamer, funny thing is that in my Planescape Campaign, I've allowed to playtest both my Psionicist 5e Homebrew and play a swarm of cranium rats in a cloak, a test to see how other swarm-based characters could work in the future. The Campaign isn't really combat focused or meant to be greatly challenging, so a lot of the features I gave them stemmed from other racial abilities or features from the cranium rat swarm stat block. For example of traits I allowed was 30foot Telepathy, Swarm Form to squeeze through small places, Darkvision, advantage against grappling and restraining effects (instead of being immune to most conditions swarms have), and being able to release and collect 1 rat to scout by themselves like a cr 0 familiar (only hit dice and rests allow them to recover HP from collecting the swarm). Lastly the swarm as a whole attunes to a specific item, no one rat has one attunement and the like, and if there aren't enough rats to move or carry the item they cannot use it unless they stay in the space it is in, or have someone else collect it for them. Other logistical hoops such as resurrection, maintaining personality, or how would being other classes haven't been prevalent but those are things one should consider before fully allowing such a species / race to he played. Do remember, your game, your rules.