Should i tell my vet that my puppy is a chihuahua mix or a poodle mix?

I doubt what you tell him she is, is going to make much difference - he'll make up his own mind (probably wrong if I know vets!!). He's going to be far more focused on any health issues he might find, than what mixes she is, much as some illnesses are, of course, breed specific. He'll ask, if he needs to know at which time you can just tell him what breeds there are in her mix.