I have used part worn slicks(just scrubbed in really) on a 160bhp 205 gti. Being FWD there was some inherint problems. Warming up the rears was a royal pain in the backside. You actually had to slide it around to get some temperature in them. The reasons where as follows:
They were too wide(equvalent of fractionally wider than a 205/50 tyre.)
FWD and its fundamental issues of heating rear tyres up properly.
Driving a rear wheel drive car, you would probably only suffer one of those issues, and that is the issue that is easily solved!

Choose the correct width.
A 205 slick is possibly a bit to wide for a 160-180bhp car such as ours fall into, but for FWD it is ideal as it balances the problems of the lack of weight you experience over the front wheels in a FWD car as you accelerate. 205 is too wide for the rear end of our 205 I am sure. We are going to try a 195/50r15 r888's on the back next and keep the slicks up front.(we just happen to have a couple kicking about.)
Mixing and matching will upset some, but in our shoddy car it might just work
I think part worn slicks will always be a cheaper and better* altnernative if you have 2 sets of wheels available to you. *But Speccing them properly will be the hard part as you are only buying what is available second hand, which considering clios have moved to 17" wheels is only going to depenish.
