is using custom skins legal

Like Sukishoo said, it's perfectly fine and legal. Riot just says to use them at your own risk since some programs may add things Riot doesn't allow or may end up downloading something they don't allow without your knowledge.

depends on what you mean by as "skin". The key thing is that it Can't give you an advantage that other players don't have vanilla access to.

Yes it's usually fine as long as it's not from a program like MKJogo (as that has other things that modify the game)

what is a good program then

no u can get swatted for that

it's fine and legal, Riot said so themselves. but it's more of a "grey" area