So I've been busy painting a set of R32 GTST rims to throw on the 2, they've come up a treat.
Currently just got a set of 6.5" rims, not sure if ill NEED to get two GTR rims?
 
I was hoping to get 205/'s up front and either 215's or 225's on the back (Federal 595's)
 
Has anyone had experience in fitting wheels and tyres of similar size? I.e what will fit?

If you must to use GTST 16X6.5 rims all round, my best guess would be to go for 195/50 on the front and 215/45 for the rears if they are available.
The 6.5 width rim is really a bit narrow for the rears, and while a 225/50 tyre would fit ok  I have doubts about how the car would handle with them.   (BTW a 225/45 tyre needs a wider rim)
 
It may be best to look for a pair of GTR 16X8 rims for the rear.
With those you could use a 225/45R16 that would be best for handling and rolling diameter.
 
You could then go for a 205 width tyre for the fronts on your GTST rims. Either a 45 or a 50 profile would work ok handling wise, but a 50 profile would ride softer.

I would advise to get,
Front: r32 gtst rim with 205/50/16
Rear r32 gtr rim with 225/45/16
 
And would advise against anything lower than those widths as they were standard widths. And keep the ratio 20 apart.
eg. 215 and 235 or 225 and 245. 
 
Hope that helps you out.

cfan I ask, why bigger profile at the front compared to rear?

Agree with the above comments, the GTR rears would be a better option.
 

Gatesys SW20
cfan I ask, why bigger profile at the front compared to rear?

Keeps the rolling diameter closer front/back, similar to standard. Profile is a percentage of the tyre width and not a fixed value across different sizes.

oh yeah, I see.
Fair enough, So not a good idea to put a 45 profile on the front?

Gatesys SW20
oh yeah, I see.
Fair enough, So not a good idea to put a 45 profile on the front?

I found that 45 profile tyres on GTST wheels on the front work fine,  but ride noticably harder than 50's.
 
Another minor factor is that with a 205/50R16 on the front on GTST wheels that have an offset of 40mm compared to 45 mm for the stock wheels, the scrub radius will be closer to stock because those tyres will have a one inch larger diameter than the OEM diameter.
 
Rolling diameter is more important on the rear since it affects gearing and speedo error, and a 225/45/R16 rear tyre would  give very close to stock RD.