Game iterates over all defined grabs, filters them, and selects 8 at random. The randomly generated characters are currently excluded from generating RESTRAINT grabs.
unitdatas_versus_mode.json is not used for random characters. Character randomization is not configurable by json. You might be thinking of random_char_cards, which is used to help generate more handcrafted outfit sets. There is no equivalent of random_char_cards for random grabs.