G'day RAM750,
No regulatory reason at all why you cannot do a CSU/RG endorsement whilst holding a GFPT. Remember the endorsement is for special design features and not aircraft type. So provided you meet the experience requirements (mainly for insurance) and can display an ability to handle the aircraft you can legally fly something like a C210 or Bonanza. Without wanting to sound patronising though , whilst you're still in a steep learning curve finishing your PPL you would perhaps find these types to be more than a handful.
Of course common sense has to prevail here and you should stick initially with the more docile aircraft like P28R or C172RG. I'm sure your flying school would recommend the same.
I know a guy who had only just gained a GFPT and bought himself a very nice Arrow 200. Unfortunately he didn't perservere with flying and never quite got comfortable with the aeroplane. Point I am making is that he was able to jump into his arrow and blast off into the training area. He did fall for the trap though that a lot of Arrow pilots do, that is he had the nav lights on during the day which dims the landing gear down lights to the point of them being almost impossible to see any illumination. A lot of the twr/ATC folk are aware of this one and first question they ask when an arrow pilot declares "pan pan" with landing gear failure is to "check your nav lights"
