Basically ive got a spare set of 17" alloys, will they fit okay on my Haynes style locost in building or are 17's a bit on the big side?

depends if you want it to look good, or go round corners quick

4" too big

Don't do it!

Stick to 15" or 13" if you want decent tyre choice, or 14" if you want something in between albeit with less tyre choice

Thought that might be the case, ill stick them wheels on my civic then haha. I suppose i could then use my civic 15" wheels as they are a 4x100 fitment and in guessing if i use either the ford ir mazda hubs they should fit fine.

Ford is 4x108 so unfortunately you won't be able to use the wheels unless they are double fitment drilled

Assuming book rear uprights then the smallest you can go is 15"