Like, in HG, you can find maylene in celadon and wake near the safari zone.
Ah, It would be cool, but no.
Although, you CAN find Cynthia in Undella Town, but she is very difficult to beat, she uses pokemon in the mid to high 70's.
But you won't find any gym leaders, sorry.
no you cant only Cynthia who is the Sinnoh champion am i correct?