Apparently it's a lot faster and they go into full armour from the get go.
They are also implanted with combat tactics and general knowledge memories.
Again, the implantation order for Primaris from the Index Astartes is the exact same as a standard Marine's, with the extra organs added into the same time frame. It takes exactly the same amount of time as any standard Marine, per the text, and they can't go into full power armor from the get-go, because they don't get their Black Carapace until year 4-5ish.
Marines are implanted with combat tactics and general knowledge memories. Primaris are not
unique in that regard.
The current Codex: Space Marines says the following:
The newly rewritten Codex Astartes allows for each Battle Company to be reinforced with additional troops reassigned from the Reserve Companies. These seconded battle-brothers typically form squads XI upwards. Once attached to a Battle Company, it is standard practice for the newly joined reserve squads to take new markings corresponding to their new company, although the rapid nature of war does not always afford time for such a formality.
Honestly, the Reserve company reinforcements seems like a good place, but the C:SM shows Primaris Aggressors as Fire Support Squads, but on their datasheet, they have the Elite slot icon as well. I wouldn't necessarily take the game rules slot the unit occupies on its datasheet as the same as the functional "fluff" location that such a squad would be utilized in.
So I think you are very free to simply put your Reivers into a Battle Company slot previously occupied by an Assault squad if you want.