Rockstar has said that the HD universe (GTA IV, TLAD, TBoGT, and GTA V) is in a different universe and therefore is not in the same canon as the 3D (GTA III, San Andreas, Vice City, etc.). That is why CJ or any characters from the 3D universe will be in this game.
I heard too that the universes are separate but what I don't understand is how OG Loc and Mad Dog are in it? If that was the case, that is.
Universes are separate as others (meyersry in detail) have mentioned, however brands (be it cars, restaurants, bands, gangs, etc) cross-over thus the OG Loc/Mad Dog/Love Fist references in decoration.