if you train jumpshot guards, all players which played on a guardposition that week getting training, dont forget to give them 48 mins if possible to reach 100% training.
if he pops depends of your trainer and his subskill and all the other things which influence the training like height age potential and so on, if he doesnt pop he still got better in his subskill which increased, so he might pop the week after if you continue with the same training.
i dont know how much difference there is between training sg/pg and sf/sg probably not that much, but you could get faster training if you only train one position.
the position which is written on the player details, has no influence to anything, its just a little help to see on which position he is best at the moment.