Is it unusual that my zangoose and seviper are being friends despite what pokedex says?

Not at all, actually! While it’s true that wild zangoose and seviper are blood enemies, once tamed and given proper homes (with properly established feeding routines), you can actually eliminate any need for either of them to fight. It’s much like how most people believe that growlithe, meowth, and marill are all mortal enemies, but in actuality, all three lines can get along just fine if introduced to a loving home … and, well, taught not to attack each other.

It’s wonderful to hear that your pokémon are getting along splendidly, though, anonymous, so may their relationship thrive for years to come!