Yes, MINIMUMS is a Scrabble word!

Letter values in Scrabble US: M 3, I 1, N 1, I 1, M 3, U 1, M 3, S 1.

Is it a valid word?

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

the smallest possible quantity
the point on a curve where the tangent changes from negative on the left to positive on the right

Also: lower limit.