The ability states that the zombie/skeleton has HD equal to your Oracle level. It is summoned, not created, so it simply materializes. That would suggest that it doesn't matter what the base creature is - your choice - and it comes into play with HD equal to class level. Advanced is a template found on page 294 of the Bestiary.
At 7th level you should be able to summon a fast zombie hydra with 7HD.
EDIT: For the record, this actually reduces the HD of the Hydra. A hydra zombie of any type is supposed to have 9 HD (because it is Huge). Personally I would rule that until you can summon a 9 HD zombie, the hydra would be off limits.