Nicolaitanes is the plural.
Yes, it can be found that way and as Nicolaitans, plural, and Nicolaitan, singular. I think Nicolaitano is an error spelling, and least that’s what searches indicate.
FWIW, this is what wikitionary entry:
English
Alternative forms
Nicholaitan
Etymology
From Nicolaite + -an.
Noun
Nicolaitan (plural Nicolaitans)
A member of an early Christian sect, said to participate in certain pagan ceremonies.