Thinking of gettin another set of alloys with track rubber on them.

Will different size or weight alloys mess up the geometry settings of my car?

Not if your track tyres are radials.

If they are much wider than your current wheels/tyres you will fall foul of the wheel arch clearances etc. but i think you know this.

so am i rite in saying as long as the alloys and tyres and smaller or the same size then eveything is fine?

Yes.
Remember that alloys have specific sizes, and 17 7 is not the end of it,
The rims have an Off-set dimension too, such as 43mm.

It is important to fit alloys to the Impreza where the off-set is the same or very close, ie 43mm can be replaced by 45mm.

The off-set of your current wheels is cast into the rear of one of the spokes of the wheels.

Thanks for the info 911. Gr8 as always.