>>40296953In my opinion, you need to have completed one of the original pairs, and the third version / sequel to be able to say you really beat that generation / experienced that region.
On the very first playthrough of the pair, it is poor form to box the starter during main-game. Subsequent replays allow it.
On no serious playthroughs should you use old Pokemon.
On the third version, neither of these restrictions apply.
Overlapping types are allowed for all playthroughs, but multiples of the same species should be discouraged.
How's this for a guideline?