Page 79 of V20 clearly states the numbers that you are looking for in a text box called "Advancing New Characters":
Advancing New Characters
Storytellers may choose to allow players to create more experienced and knowledgeable characters. Indeed, players of Vampire: The Masquerade 20th Anniversary Edition may prefer an “ancilla game” or an all-elders chronicle instead. In this case, we recommend first building a neonate character and then granting players a number of experience points that allows them to increase their characters’ Traits to levels suitable to the chronicle and the age of their vampires.
As a basic rule of thumb, “idle” Kindred should have a number of Discipline dots equal to the square root of her age. Remember that players’ characters are rarely “idle” like Storyteller characters, so they’ll rapidly outpace this guideline. That’s fine; they’re out there in the world, having exciting encounters and earning more experience than passive Storyteller characters. Remember that the cost for raising a Trait which is already advanced can be very expensive.
See p. 124 for more information on spending experience points.
Kindred Age Category: Experience Points
Neonate: 0-35
Ancilla: 75-220
Elder: 250-600
Methuselahs: 1000+
I would think of this "rule of thumb" as just a limit of how many disciplines may be bought with XP. For example, if a typical Camarilla fledgling generates with 3 disciplines (up to 5), let's say that he has spent 80 years as an "idle" vampire and has 150 bonus XP. He could theoretically buy a lot of discipline points with that! But is limited to a square root of his age, which is ~9. And he may only buy up to 6, as he already has 3 that he got during generation.
If the Storyteller assumes that the character was not "idle" but rather as active as typical player character during that time, the Storyteller is free do provide bonus XP and no limit on discipline dots.
Actual benefit of Kitsune
The paragraph you quoted from W20 Changing Breeds says that:
- Kitsune can "learn potent Gifts of their own", without a teacher.
- Kitsune can "learn the Gifts of other Changing Breeds" (any of them), provided that they find a teacher.
The experience cost is specified as "the same as for a non-breed/auspice/tribal Gift", and we don't know if it applies to both points and Kitsune have to pay increased cost if they learn a "potent Gift" on their own.
I do not know if “potent” is just a fluff word here or a game term; as it is not capitalized, I would assume the former.
Let us compare it to the “standard situation of W20” — to the Garou.
Garou, according to the Learning Gifts paragraph on pages 151-152 of W20 Core, always need a mentor to learn a Gift. If it is a Spirit, the process goes fast, if it is another Garou, not so fast, but still possible. Breed, Auspice and Tribe Gifts cost 3*level XP, others cost 5*level XP.
Garou do not get the opportunity to learn the Gifts of other Changing Breeds (other Fera). At least I was not able to find something that allows them to do it, though neither was something directly forbidding it found. The sentence "a werewolf may learn the Gifts of other breeds, auspices or tribes" implies not being able to learn Kitsune Gifts, but rather Gifts of Homid if you are Lupus, for example.
So, in W20 the benefit of Kitsune is not “quantity of Gifts” (the ability to learn more Gifts from their list with the same amount of XP), but rather “quality of Gifts” (the ability to learn more types of Gifts and easy access to Kitsune Gifts). This is further supported by the phrase "Kitsune rely heavily upon their magical strength and versatility."
White Wolf books are flawed
The books printed by White Wolf inherit a lot of text from each other, and when rules (or lore) change from edition to edition, some now unrelated or obsolete text may occasionally stay.
For example, V20 Core, p. 107:
If you don’t have any dots in a Knowledge, you cannot
even attempt a roll involving it unless the Storyteller
gives explicit permission (such as where common trivia
is concerned). If you don’t know Spanish, you can’t try
holding a conversation en español on your Wits alone.
But Linguistics is no longer a Knowledge in V20, languages are 1-dot Merits, and you either know it or don't know. This probably still explains how do Knowledge checks work, but the example check is irrelevant. If they changed it to, for example, a Computers check, it would be a lot more relevant, and in addition it would explain what does it mean to have 0 dots in Computers: not being able to use computers at all, or only to hack into the systems?
Or, regarding W20, see the question "How does Calm Heart work?": a Merit from VtM series was directly reprinted without fixing the issue with vampire and werewolf Frenzy functioning differently. You would not be able to use this Merit out of the box, you would have to home-rule some other way to make resisting Frenzy easier or find another book to use.
And the most iconic example being Page XX.
This makes understanding WW books... hard. Sometimes. At least do not expect it to always be easy.
GM fiat is what really limits you ever
You asked about the GM fiat -- here is my answer.
Having to find a teacher to learn a "foreign" Gift effectively means that GM can deprive you of any Gift. He/she may also deprive you of any other option you want to take, just because "Rule 0" gets reprinted in WW books over and over and remains essential, allowing to rule however your ST wants to rule. This is especially important regarding the ability not to look for a teacher when learning a native Kitsune Gift, my inner oWoD player senses tell not to expect every ST to actually allow learning any Native Gift that way.
Whatever is written in the book, all of the GMs that I met had something work very differently compared to RAW, sometimes intentionally (they did not like how it is done by WW), sometimes just because they didn't bother to learn all of the rules. And I also noticed that the more people call some Storyteller I know good, the more are the rules of that Storyteller different from RAW. That is a thing to consider.
Just because it is (un)fortunately not Pathfinder, even if it is possible to do something in oWoD according to RAW, one should be totally ready hear a "no, just no" from the Storyteller.
Best Answer
There are a few issues at work here:
It's not balanced, or at least not with the hair-trigger fineness that, say, D&D4 attempts to achieve. The characters are not meant to be equal, they're meant to be comparable. (Ultimately so much of what a Garou can do in a fight depends on their inherent Garou resilience, form changing, Gifts and aggravated-damage-claws that how the tribes and auspices balance is a detail on top.)
It's about area of specialty. This is the World of Darkness; fighting is important to werewolves, but in the long run it's the least important decision they make. Ahroun are combat monsters; if you're looking for combat balance between a Black Fury Ahroun and a Glass Walker Theurge, then sorry, it's not there and it's not meant to be. The Ahroun would wipe the floor with the Theurge... if the Theurge is so poor at his job that he lets the challenge get to a klaive duel instead of a ritual or riddle-game or having spirits warn him about the threat.
(Compared to any non-Garou, of course, even a Bone Gnawer Ragabash is a lethally dangerous killing machine.)
Rage, in particular, is not all good. The balance between Rage 2 and Rage 6 is that a Rage 2 character only has 2 rage. He won't frenzy, he won't panic and run in fear at a Nexus Crawler, he won't get taunted into a berserk cannibalistic charge every time the moon is full. (Combat characters that run high-rage also need to spend plenty of freebie points and xp on Willpower, if they want to stay alive for long.)
You've left out any mention of skills, but WoD is a modern-world roleplay and skills are very important. All characters get the same number of skill points, yet High Primal-Urge, for example, is a major Lupus advantage... because wolves aren't spending all their skill points on Drive and Law and Language and all those other pointless things the homids all seem so worried about.
With the exact same tribes, rage and willpower, I promise you that the fight between a Brawl 4, Melee 5, Dodge 4 Get of Fenris berserker and a Brawl 1, Melee 0 Dodge 2 Ragabash will be... one sided.
(Which serves the Ragabash right for letting it come to a fight. The Ragabash has Stealth 4, Empathy 4, Law 3. So the warrior isn't going to like the mysterious disasters that happen to him over the next few weeks. Which might just drive him into a frenzy next full moon moot. Which will seriously jeopardize his position in the sept. Being a trickster is about not needing to hit things to make your point.)
Tribe / form Rage and Willpower are only meant to be starting points anyway; both are cheap to buy up with freebie points during character creation.