I'll have to double check how the game counts Elf requirement, it could be that it's still only counting Daisys.
Alternate costumes are unlocked Via item codes which are given to Patreon supporters. There may eventually be a way to unlock them through gameplay, when more premium skins become available.
One tip for Lamia taming is to send a weak Harpy to fight a horny Lamia. When monsters win a fight there is a chance they will have sex with the defeated enemy (this is what the game considers 'rape') So if you continually send a Harpy to lose a fight against a visiting Lamia they will eventually take their prize and become a resident.
I'll keep that in mind for the requirements for Succubus, at the moment they are only an enemy species anyway and cannot yet be tamed.