just to add to the previous poster, you can do taxi missions on in the beginning but they're one-shots. you have to call roman at the cab depot for each one and they don't contribute to 100%.
as for choosing dwayne or playboy, if you kill dwayne playboy gives you cash and then disappears on you (the worse choice as far as i can tell). if you kill playboy you get his apartment in north algonquin (complete with the outfit from gta3 and a nice pool table you can't use), you get dwayne as a friend and once you get him to a certain point of liking you (75% or 80%) you can call dwayne and he'll send a car full of gangsters to help you out wherever you called from. i even had one of them jump on the back of my motorcycle and shoot at people chasing me (a completely unexpected but welcome surprise).
as for stevies missions, i've always associated them with finishing the first Phil Bell mission because that gives you the assassination missions, but i read that it's after completing derrick mccreary's first mission (where he's smacked out in a park in alderny) and makes you chase down his associate at the burgershot. after that just answer every random call from brucie and in between steroid driven rants he'll give you the missions.