The team can play unlimited number of U21 player. hence, no need to put Ngog into the senior squad list. This list of 21 players is fixed till the Jan transfer window. Even if RH had filled with 25 players, he can still change and sign more players when the transfer window next opens.
Also, this is only limited to EPL. Europa league dont have the 25 player squad limit and minimum 8 homegrown player quota.