The Scions basically told you to join them because they'll let you use retainers if you do. That is the single way in which they even remotely lived up to their promise to help you with your ventures instead of the other way around. So I'd say your retainers are top notch canon.
Or they have something up on the MB in which case everyone on my server can see their name on the MB listing, clear as day.Nobody else can see your retainer unless you own a house and park them outside it.
My FSH retainer is farming Ravana ex, donno about yours.You can't take them into dungeons.
With the exception of DRK, none of those jobs should have any trouble giving you extra Soul Crystals depending on where you are in the story. MCH is a brand new job and we can safely assume anyone with the title has a Soul Crystal just like how you do (MCH is basically a class but has a Soul Crystal because that's how this game works). SAM I dare say falls under a similar issue, but once you're buddies with the Samurai in Kugane it's not unthinkabe you can get extra Soul Crystas for your retainers should you vouche for them. BLMs we know of several active Soul Crystals, likely there are more. SCHs, just talk to your buddies in the Wanderer's Palace, etc, etc. Even for WHMs it's not that only Padjals can be WHMs, it's they're the only ones allowed to be. But we don't know if they have in their possession all the Soul Crystals. Hell it didn't look like the Seedseers even knew the Soul Crystal you have was in the tree where you found it. You could've easily found a long lost one during a venture into the Amdapori dungeons. Any other job you can basically get more Soul Crystals by talking to your mentor who gave you one in the first place.SE just painted themselves into a corner with now classes/jobs work. They either need to retrofit a reason to give them a job stone, or they need to create a precursor class for Astrologian, Machinist, Red Mage, Dark Knight, and Samurai
The only job where this logic falls the shortest isn't WHM, it's actually DRK, but seeing how you basically found yours lying on the floor in the street, maybe you found another one for your retainer lmao. Or, given that your squad-mates supposedly had adventures before joining, they could've had their own job quest already.
tl;dr there's nothing stopping squadmates from having job and many of the quests actually support this sort of thing, relying on your character vouching for your squad mates. Soul Crystals must be handed back upon discharge from the squad.