Yes, FEWEST is a Scrabble word!

Letter values in Scrabble US: F 4, E 1, W 4, E 1, S 1, T 1.

Is it a valid word?

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

(superlative of `few' used with count nouns and usually preceded by `the') quantifier meaning the smallest in number
a quantifier that can be used with count nouns and is often preceded by `a'; a small but indefinite number