лапушка - on who can it be used?

I was looking for terms of endearments and ran into this...

Can this be used on anyone like "Sweetheart" or is it limited in some way?

Yes, it is like "sweetheart" - either tender and intimate or sarcastic. No specific limitations.

In one version of the folk song Вдоль по Питерской ("Along Piterskaya Street"), the second verse includes the lines:

Эх, едет миленький сам на троечке
("Hey, [my] darling himself is riding in a three-horse carriage")
Ох, едет лапушка, по просёлочкам
("Oh, [my] sweetheart is riding along the dirt roads")

So even though the word is grammatically feminine, in this context it's clearly referring to a male sweetheart -- i.e., it's the same person as the unambiguously masculine "миленький сам".

Also, just as an etymology note: лапушка is literally a double-diminutive signifying "a wee little itty-bitty paw" -- deriving from лапа, "an animal's foot". (When I first heard the word, I thought it was from лапша, "a noodle", since "my little dumpling" can be a term of endearment in English.)

Лапушка, лапочка, солнышко, ласточка, пташечка, золотце, зайка, котик, ...