Legos refers to the lego bricks like waters can refer to multiple glasses of water even though water is a mass noun and usually doesn't exist in plural. Same with peoples meaning multiple groups of people.
Sheep is perhaps a better example for comparison with Lego. Unless you know of an example where ‘sheeps’ has legitimate usage.
not really, because sheep IS already plural. there is no singular for sheep. if there was it would have to be shoop. (feet->foot, teeth->tooth, etc...)