
In future, you can use the name generator on the game itself! (Symbol looks like a little dice, next to the boxes where you type your name in.) It can be guaranteed to differentiate between clans properly, and considering they all have their own separate naming conventions (as you've learned), that's important.
Whether you fantasia, name-change, or come up with a backstory reason is completely up to you and what you feel most comfortable with, since all three would ultimately work. One other option, though, is to just... have your character introduce himself by a different name when RPing. It's not ideal (you'll still get some people who look at the nameplate), but at the end of the day, nameplates are meta info and information given through actual RP should/will take precedence. So if your character is introducing himself by a given name, that's what most other characters are going to call him. It helps if you put it in his search comment, too (since then it can be referred to without people having to scroll up or ask you).
Lastly, for RP job - those two have pretty different IC flavours, so I would ultimately decide based on what you think suits your character best after playing/youtubing the job quests for each. But either could be appropriate for a xaela, and xaela NPC's of both exist! It's also worth noting that you can RP a character who wields a katana without being a Soul Crystal Carrying Samurai, and likewise a character who wields an axe without being a Soul Crystal Carrying Warrior. The soul crystals are only important if you're going into the more mystical, historical side of the job (haven't played SAM, but for WAR that means invoking and controlling the Inner Beast). So don't feel like you need to find a way for him to obtain a soul crystal unless that's specifically what you want - don't feel a need to force it, in other words.
Whether you fantasia, name-change, or come up with a backstory reason is completely up to you and what you feel most comfortable with, since all three would ultimately work. One other option, though, is to just... have your character introduce himself by a different name when RPing. It's not ideal (you'll still get some people who look at the nameplate), but at the end of the day, nameplates are meta info and information given through actual RP should/will take precedence. So if your character is introducing himself by a given name, that's what most other characters are going to call him. It helps if you put it in his search comment, too (since then it can be referred to without people having to scroll up or ask you).
Lastly, for RP job - those two have pretty different IC flavours, so I would ultimately decide based on what you think suits your character best after playing/youtubing the job quests for each. But either could be appropriate for a xaela, and xaela NPC's of both exist! It's also worth noting that you can RP a character who wields a katana without being a Soul Crystal Carrying Samurai, and likewise a character who wields an axe without being a Soul Crystal Carrying Warrior. The soul crystals are only important if you're going into the more mystical, historical side of the job (haven't played SAM, but for WAR that means invoking and controlling the Inner Beast). So don't feel like you need to find a way for him to obtain a soul crystal unless that's specifically what you want - don't feel a need to force it, in other words.