Does double weakness count ?

In the main games, pokemon can have 2x weakness or 4x weakness, but in this game they can only have weakness applied once (though the multiplier is 1.25x instead of 2x). Weakness will multiply with STAB though, so the max is 1.525x.

Immunity is scaled down to resistance, which is scaled down to .8x in this game. Whether or not that makes Aerodactyl receives Ground moves 1x, I don't know (I think Immunity should at least be double penalized to negate the effect of the other type being weak).

This is the late answer, but based on latest research, weaknesses do stack. Jolteon's attack do x1.25x1.25 on account of Electric being effective against both flying and water.