Could I use gryph-chargers for my Warhammer 40k space marines? And would they fit?
While 40K and Age of Sigmar are both technically 28mm scale models AoS tends to run a little larger than 40K.
That said the Stormcast models are a very close match to Primaris models in terms of size, so a waist up conversion for Vanguard-Palladors is possible.
Otherwise if you're wanting to use regular marines either the legs will look a little off, or they'll need more heavy conversion.