I was hunting for the relevant thread since I think something similar got asked not too long ago, but I seem to be selectively blind and cannot find it. Hopefully someone else will see it and provide a link, since I think it included a fan-made "tier list" of job-class versus how believable playing one is.
... Mostly because how "easy" it is to play a Job in RP is going to depend on the job itself. White Mage, as you said, is kind of restricted due to only the Padjali being the only real "practitioners" of it save for the Warrior of Light. Same with Dragoons now that the number of active practicing Dragoons was listed in the lore book. Conversely, however, you have Paladins - which include the Sultansworn, along with a bunch of Free Paladins who just go around helping people. You have a whole retinue of Machinists being trained up in order to modernize the Ishgardian military.
However, even with the more "restrictive" jobs... there's ways you can explain away why your character has those skills without horribly breaking the lore. The Dragoon list is for active duty members, it doesn't include those in training or those who may have retired. Your character could have stumbled across a soul crystal and writings about Succor while delving into an Amdaporian ruin.
Even with Red Mage, there's another tack you can take with it. The RDM quest giver is the last of the "Crimson Duelists," a specific group of RDMs who aided the Ala Mhigan forces. That doesn't mean there aren't other groups of RDMs out there. And, on top of that, he's been pretty open in his teachings - considering he trains up three different people by himself in handling Red Magic.
So... there really isn't a definitive answer. What jobs you can "play" ICly depend heavily both on how you explain it for your character, the group you're RPing with and how far they're willing to bend the lore, and just how restrictive the job is based off the lore to begin with.
... Mostly because how "easy" it is to play a Job in RP is going to depend on the job itself. White Mage, as you said, is kind of restricted due to only the Padjali being the only real "practitioners" of it save for the Warrior of Light. Same with Dragoons now that the number of active practicing Dragoons was listed in the lore book. Conversely, however, you have Paladins - which include the Sultansworn, along with a bunch of Free Paladins who just go around helping people. You have a whole retinue of Machinists being trained up in order to modernize the Ishgardian military.
However, even with the more "restrictive" jobs... there's ways you can explain away why your character has those skills without horribly breaking the lore. The Dragoon list is for active duty members, it doesn't include those in training or those who may have retired. Your character could have stumbled across a soul crystal and writings about Succor while delving into an Amdaporian ruin.
Even with Red Mage, there's another tack you can take with it. The RDM quest giver is the last of the "Crimson Duelists," a specific group of RDMs who aided the Ala Mhigan forces. That doesn't mean there aren't other groups of RDMs out there. And, on top of that, he's been pretty open in his teachings - considering he trains up three different people by himself in handling Red Magic.
So... there really isn't a definitive answer. What jobs you can "play" ICly depend heavily both on how you explain it for your character, the group you're RPing with and how far they're willing to bend the lore, and just how restrictive the job is based off the lore to begin with.