You have to pick a team leader, sprinter, kom, 2 allrounders and 4 domestiques. You can only choose two riders from the same team. Like Velogames, you have a points limit so you can't just choose who you want. You can stay with what you pick and not alter it and this means you will stay in the purists league. If you make a transfer you will no longer be in the purists league.
Generally you get about two transfers per stage. You can use these to replace riders who drop out or you can use them tactically at the end of each stage to transfer in riders you fancy for the next stage subject to you having enough points. Careful though, you can easily mess up your team by making too many tactical transfers.
Throughout the game the value of the riders changes slightly according to how they are doing. Well and they increment up, bad and they increment down.