Pretty sure you can learn moves from any level of mob as long as its that particular mob family (Like you can only learn Head Butt from Mandragoras, but you cant from Quaduvs which use the exact same move).
So in theory, you should be able to learn the fomor moves from the hydra corps as long as you meet the requirements.
It's not a matter of level, it's a matter of being able to learn both spells from the same mob type in a specific place, considering usually you have to get them from mobs in very different places, that don't usually share moves.
I'm trying to figure this out for the sake of the
draft of a mini-guide I'm putting together on the order in which to get your spells, taking into account you can get more than 1 spell from some mobs families, so for people who'd rather level to 99 and then learn their spells, or at least take a similar-enough approach, the necessary travelling would be minimized by only learning from a mob family when you can learn the highest-level spell from that mob type - that way, even if you fail to get one of the spells you might get the other.
The shadow/fomor issue throws a bit of a wrench into these gears, since I'm yet to find a report of people learning the corresponding spells from the only mobs I'm aware of that seem to have both sets of abilities...