Well, for friendly (semi) Megabeasts, you could add the [POWER] tag and hope they take over a friendly human civ in worldgen. That's really the only way to get a non-hostile Megabeast short of taming them.
As for the sorcerers you've described, you really don't need a new creature at all save for descriptive purposes. It's just a matter of having a SECRET interaction which grants a bunch of other interactions and other goodies. Those evil sorcerers would need a raise corpse-like interaction to revive and transform creatures into the specified minion you want and/or build towers, and would have to be immortal so their servants don't kill them as soon as they show up post world-gen.
Oh, and for these guys to be exclusive, they should not be able to teach or pass on their secrets to others, though there are ways to let them "teach" others part of the secret without gifting them their most powerful or even entirely related features; you can, for instance, have a sorcerer "teach" a prospective student part of the secret, only to have the student transform into some huge monster that serves as the sorcerer's henchman from then on, all-the-while leaving the sorcerer untouched by this transformation. However, for this specific purpose, you might indeed be required to have a few unique creatures so part of the interaction only/doesn't affect the sorcerer.
So yep, it's doable.