>>48592554Turtwig will get the same amount of EXP. regardless of how many Pokemon you have in your party.
The battling Pokemon doesn't receive an EXP. penalty when using the EXP. Share anymore, and hasn't for a while.
The best way to avoid overleveling is to not use the same Pokemon for every battle, anything in slot 2-6 in your party gains 50% exp, so naturally some are bound to be underleveled based on their level rate. Use them and let your stronger Pokemon gain scraps of EXP.
Remember that the EXP. gained by reserves is still dependent on their level compared to the defeated Pokemon, so not overleveling is very easy if you simply don't use only your starter the entire game.
