how to take unfinished/lost tales books...

I've always wondered whats the General level of acceptance for the stuff it says in there. do most people take it as fact or not seeing as it wasn't the finished version of anything

for instance the mention of a certain "Legolas Greenleaf" in the fall of gondolin chapter. we know this can't really be the Legolas we all know and love seeing he was the son of the king of the wood elves and there was no reason for him to be hanging out with all the gays in gondolin.

or all that junk about the cats and huan in the tale of luthien and beren. did that actually happen or was it cut out of the story or is that an add in to the story?

anyway if you get my point gimme an answer