you're supposed to do the tobi but i just use whatever neb cups I happen to have at the time. I don't think it should make too much of a difference. I don't like the pari because the flappy thing sprays sticky medicine up into my face and it takes longer than the regular one. I don't think using a different neb cup would make too much of a drastic difference, but pari is whats recommended.
i do it after everything because then all the mucus and stuff is floating around my lungs to get attacked by the tobi then I can cough out whats left. does he do HS with the vest? If I do it with the vest I find I don't cough as irritably so maybe he won't be coughign as much during tobi. also, sometimes when my nebs make me cough to much I do an albuteral inhaler 15 minutes before.