I might add Horde and Mob as other viable name options for groups of small and larger creatures.
I’m working on setting up a campaign so I can’t say I have. Depending on how detailed you are looking to get, you could try converting or adjusting the following D&D 5e UA rules as well as the associated DMG, p250.
It includes things such as morale, commanders, and PCs within a unit.
PCs could be their own unit or commander of a unit.
You might consider adopting a horde/mob/troop/swarm of minions as well.
In terms of what you’re aiming for, isHP = number of creatures in unit about the gist?
Personally I think that value would be too low. You might try HP = number of units * lowest HP value of individual or use the HP of a minion version. The unit could gain adv = to the unit’s commander’s Presence attribute and for every boon or bane affecting it you could give dis/adv±1. That’d be an easy solution for morale and boons/banes. (Perhaps healing would be an exception?) Certain feats could increase the amount of dis/adv (e.g. multi specialist feats).