Associations depend on the deck/Fool is simply Air in RWS
As Ravenest showed you in your other thread about the cards and astrology, there have been many astrological associations assigned to the cards by many different tarot scholars and deck creators. What this means is that you have two choices. (1) You can pick your favorite associations, the ones that strike you as right (be they your own or someone else's), and stick to them no matter the deck. Or (2) you can consider the deck you're using and what the deck creator thinks. I usually think it best to listen to the deck creator(s) as they devised particular images based on such things, and such images won't make as much sense if you impose your own astrological wants/needs on them.
In RWS the Fool is Air because the creators of this deck were "old school" in their astrology and didn't use either Uranus or Neptune, even though both planets were known by then. RWS, instead, uses the Astrological bodies: Sun, Mercury, Venus, Moon, Mars, Jupiter, Saturn, the twelve zodiac signs and three of the four elements: Air, Water, Fire. In more modern decks, Uranus is considered an acceptable substitute for the Fool as it is related to "Air." However, you must remember that the cards ruled by planets do not also have a sign attached to them. Mercury rules the Magician, not Gemini. Neptune rules the Hanged Man, not Pisces.
This is because other cards have ownership of those signs. The whole idea of including planets and elements was to make sure that the cards didn't *share* signs. Otherwise they could have just said the Fool is Aquarius and so is the Star. But they didn't so that. So, if you're using a deck like RWS, and respecting the astrological assignments of that deck, then the only Aquarius card in the deck is the Star. Not the Fool.
Once again, however, if you feel strongly about certain signs belonging to certain cards then that is entirely up to you and you can see them that way. But if you're asking us for the bottom line "rule" on the subject, then we have to say that, in RWS, the Fool is simply "Air." He is not Mercury in RWS, and he is not specifically Aquarius. He is, purely and simply, about the element of Air, and could indicate anyone who is have a day where air signs dominate their lives.