Yes, SCALIEST is a Scrabble word!

Letter values in Scrabble US: S 1, C 3, A 1, L 1, I 1, E 1, S 1, T 1.

Is it a valid word?

SCALIEST is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

rough to the touch; covered with scales or scurf
having the body covered or partially covered with thin horny plates, as some fish and reptiles

Not playable in Scrabble US: SCALEY.