I think there will be fossils. We have to get them one way or another in Alola. If they are new fossils, I think they will be water based or flying based. We do not know how long Alola has been around. It is possible that it was around when the continents were together and when they all separated, pokemon got trapped on the islands. And I am sure at one point all the regions were once together.
If not new ones, then it is highly highly possible to get Alolan Kanto Fossils, all in which are water and flying, although I do not think Aerodactly will get one.
And I highly highly doubt Rockruff is the fossil pokemon. Some pokemon are already extinct that we have and they are not fossil pokemon.