Legal?

No reason a pitcher couldn't wear it. It's just a tee shirt worn under the uniform top.

Three quarter length sleeve also available. Different color sleeves and white sleeves. IIRC, a pitcher can only wear white sleeves if the entire team does?

There are two types of tops that teams wear. One is the traditional short sleeve top and the other is the vest type.
For the short sleeve top the pitcher can wear any color under it he wants but if the undershirt sleeve is exposed it can't be white or gray.
For the vest top the undershirt can be white or gray for the pitcher if that is what the rest of the team wears but it can't extend past the elbow for the pitcher. The undershirt for the vest type top is considered part of the top.

thanks for the link....I just bought one for my son...