Well, I know that you can find Reath towards the end of the game. Reath and her partner in crime, Ferma, will eventually escape prison, and they will go to Miror B.'s room in the Pyrite cave, and you can fight them there, and snag their shadow Pokémon. As for the trainer with Aipom, I think that you can fight him again when you return to the Shadow Pokémon Lab, after you beat Evice at Realgam Tower, and explored the Snagem Hideout. As for the other trainers, you will be able to fight them again, I am not 100% sure, but you will probably be able to find them at the Snagem Hideout after you beat Evice at Realgam Tower.