DtrQ: no, you'll just add a 'solid behavior' to it, not a platform movement, they're different things. I don't see any reason it'll reduce performance either.
Having a 'solid family' is a good idea, but I think everyone's forgetting a limitation on families: due to the way family events work, you can only have one plugin type in a family (e.g. all sprites, all tiled backgrounds, all text objects). So if this limitation is kept, you can only make either sprites or tiled backgrounds solid - not both! If this limitation is removed then there's a big question about what happens with the events. I'm also not sure what effect it would have on the runtime, which in many places assumes there's a single plugin type in a family.
Something else to consider is we're planning a new model for families when we get round to them. Objects will inherit the variables and behaviors from the families they're in. So instead of family variables and behaviors being all-those-in-common, you define them seperately and all objects in the family get them, no questions asked. In this case, you can add a 'Sprite solids' family with a solid behavior, and any object in that family also gets the solid behavior. This way you get both! You can make an object solid either by adding the behavior, or adding to a family you made with a solid behavior.
With that in mind, does this sound better?