The Xindi might be everywhere, really. Their culture consisted of multiple species, was governed in a somewhat repressive manner, and (largely for the latter reason) managed to remain ignorant of the existence of at least one further, now extinct species. Several other "member species" might have been discovered later on, accounting for the Xindi Starbase of "The Battle" and for the Feline Xindi of "Slaver Weapon" etc.
Kzinti. A totally unrelated species created by Larry Niven for his Known Space
universe. Not Xindi. At all.