The remake expects you to have played the OG, Crisis Core and Dirge of Cerberus and watched Advent Children. For example Cissnei from CC is introduced in Rebirth's Gongaga and there's scene of her almost telling about Zack and Cait Sith almost revealing she's an EX-Turk. I guess you could play Remake and Rebirth without having played the OG and the others but you would be missing so much stuff. Also in her sidequest Aerith and her cook together catering for Zack fans showing Aerith and Cissnei with whom Zack flirted with doing housewives things