They come 500 years later, he's talking about the future. He obviously believes Nanaki will find more of his race. Actually, thinking about it, 500 years is too great a number in a way. He is around 15 or 16 by his tribe's standard when he is 48. That would mean 500 years later he is 167 by human standard. Oh well. Maybe humans err... live longer on Gaia haha
I don't usually nitpick that sort of thing with a plot... but there it is.
Not that I accept a damn bit of the Compilation as canon, but apparently in one bit of extra material a female of his race is talked about? Vincent and him have a chat about how they'll both be alive in the future in one of them too, and I think it directly references the ending as Nanaki coming to visit Vincent in the Midgar ruins.
I've always held a fan theory that something about Holy's influence at the end depetrified Seto and maybe there were other stones too or something?
As for the age thing, it's possible that the rate of aging between human and fireliondogs changes as the fireliondogs get older. I know that the rate of age equivalency varies on dogs, the seven = 1 year thing only applies to part of the lifespan, and I think they age quicker when they're younger so that would apply here too.
Not that I think this much real thought was put into it by the writers, it just sort of worked out. XD