What issues may I get alley cats?

Depending on what the mounting face is like they might not clear the calipers. They're usually made for Mk1/2 Escorts in that size with the disk behind the hub.

Getting suitable nuts to fit metric studs on whatever variety of alleycats you end up with could be awkward too, although I expect mk2 escort stuff would work

The fact that they are only 13x5.5 will mean you'll really struggle to make them look right - that's basically the same size as mk3 fiesta base model steelies so to keep the overall tyre radius roughtly right you'll need to use pretty big tyres. If you want to use a "sporty" tyre in a 55 profile or whatever, the rolling radius will be tiny and the wheels will look completely lost in the arches unless you lower the car a lot.

The difference in offset will make the steering feel strange and highlight any worn bushes in the front suspension (E.G lower wishbone bushes that are always halfway dead on fiestas)

I think ill scrap that idea. Thanks again for the concise and easy to understand information.