You can join as many as you have the time and money for. Don't rush into them too fast though. The appendant organizations (YR and SR in this case) build upon the blue lodge lessons. A firm grasp of your first three degrees will be essential to your enjoyment of the appendant bodies.
The sole reason I went into Scottish Rite so soon after being raised was to take advantage of a lower fee for being under 31 in my Valley (I'll be 31 this summer). I will certainly have a lot of study to do to grasp all of what I'm being shown. While yes you can join all at once, one after the other, etc, I recommend spending time in your blue lodge. The appendant bodies will still be there when you are ready.
