Answer #1 : Players need to be on relatively even terms. Disparities in XP levels just breeds saltiness. And at some point somewhere, there should be an xp cap. 900xp is just insane. Like, for me, that's not even from a "omg so strong mechanically!!!!!" kind of angle. It's just...goddamn, you can literally buy every fucking thing imaginable with 900 xp. Your character is a mary sue on steroids who can do everything. That game isn't going to be fun.
Give people enough xp to be AMAZING at their thing. 400, 500, 600? Who cares, go for it. Don't give them enough xp to be amazing at every fucking thing.
Answer #2: All you need is Staff who is willing to tell people NO when they want to do something that isn't done with people's fun in mind. If some 900 xp mage gets mad at a 900 xp werewolf, if the mage shows up to a scene and goes "i atomize the werewolf", you just say no. Find another way to work it out.
If there is some retarded nonsense going on like the mage wants to kill the werewolf because the werewolf tasted the mage's girlfriend's breastmilk or some crazy shit (thanks, Fallout MU), you just siteban all 3 people on the spot and move on.
The problem with a game like Fallcoast and obnoxiously high XP, is that there might as well be 0 oversight. Staff on TR and FC actively does not WANT to be involved in their game. They want everybody to STFU and go play in their corner and leave staff alone so they can go play in their corner. Like, dating back to when I actually played TR, you had Shane (vampire) running around with some silly effect 24/7 and going "HAHAHA I CAN'T DIE! MY HEART IS CUT OUT AND IN A BOX SOMEHWERE!!!! FUCKYEAH KINDRED VODOUN!!!!!" and people making Septemi characters specifically to kill him (being the vampire bloodline with a "dispel magic" power), people starting fights in Elysium with 0 consequences, people unfreezing coteries and instantly being given Prince, etc, etc, etc.
The answer is always going to be "is your staff any good?" XP tiers are irrelevant. With 10xp or 1000xp bad staff is bad staff and bad game culture is bad game culture.
Just because people "can" theoretically do things, doesn't mean staff should let them, on the game. And no, staff doesn't need to specifically HR out every little thing, they just need to be paying attention and willing to say no in specific cases, and not be afraid of temporarily upsetting players.
TL;DR Make xp tiers available to everybody. The only requirement should be "hey, can you like...put a little more effort into your character's story, and make sure they make sense? No 18 year old mages with 500 xp and a mastery, please." And please, put an actual hard cap somewhere.