Should slim and skinny be kept as separate tags, or should one be aliased to the other?

Based on the definitions of the tags and their aliases, they should be separate. Slim being someone that is lean, but not underweight. Skinny seems more like someone who is just skin and bones, severely underweight, maybe even malnourished. Unfortunately they seem to be used interchangeably in some cases.

Additionally, should they be added to the "Scale of fatness" present on the overweight, slightly_chubby, etc. pages?

Examples: