Yeah, I'm...very much in the same boat here. The response Oofta gave comes across as "of course any given Dwarf is going to have a harder time convincing any given Elf," which...no? I have no idea why that would be required.
I literally don't care that it isn't called "find animal companion." Its function is to provide a spirit in the form of an animal; the player is free to literally never use it as a mount if they wish. In this case, I would certainly say that the spirit could not carry the centaur-PC as a mount, because centuar PCs are too heavy, for exactly the same reason that I wouldn't let a dragonborn who weighs 350 pounds naked ride a mastiff, even if she chose to summon one with the spell. All its other functions would work normally, and because I'm not a jerk, I'd let the line about self-only spells applying to the steed "when it acts as a mount" apply if they were sharing a space, not solely when mounted. (But two creatures sharing a space have other problems, so this benefit wouldn't be used very often, I suspect.)
Having a single spell be the reason to disqualify a character from an entire class, and thus a reason to disqualify an entire race from play at all, is a bogus argument. If a DM deployed it on me, I would definitely call them out for obviously going out on a limb solely to block it. (I doubt this would ever happen, because I don't really care much about centaurs, but still.) The argument is exactly analogous to saying, "A dragonborn innately has draconic blood. This means all dragonborn would have to be draconic sorcerers, but a race that is restricted to only one class makes no sense. Therefore, dragonborn shouldn't be playable characters." Both are equally ridiculous.
Uh...yeah. Maybe not the best example, even with whatever emoji that is, given how incredibly frustrating it is to have people sincerely argue that that's how Warlords work. Also, bringing a potentially even MORE controversial discussion into the thread is very much "it's a bold strategy, Cotton, let's see if it pays off for 'em."