I don't know what City's injury problems are, but the starting 11 and the (up to) 7 subs have to be chosen from a squad of 25 registered with the Premier League at some point. Pep wouldn't be allowed to put anyone outside this squad on the bench.
You can have an unlimited number of under 21 players though. Burnley had a 19 year-old who has played about 10 minutes of premier league football on their bench. Guardiola could have done the same if he wasn't a cockwomble.
I don't know what City's injury problems are, but the starting 11 and the (up to) 7 subs have to be chosen from a squad of 25 registered with the Premier League at some point. Pep wouldn't be allowed to put anyone outside this squad on the bench.