Both the tyres on the same axle must be the same size, though the front axle can have different sized tyres to the rear. You can use a different size wheel temporarily (e.g. in the event of puncture) but you are then restricted to 50mph.
It doesn't matter if you mix wheel types (steel or alloy) on the same axle, though both must be the same size (width, diameter and offset) and fitted with same size tyres, as above.
The "Space saver" is deliberately thin and skinny as it is only supposed to be fitted temporarily, so they make it as small, thin and cheap as they can get away with.
The tyre is generally "H" rated (up to 130mph) so speed itself will not wreck it (you could theoretically run 4 Space savers!) it's just that your car is assumed to become unstable and dangerous above that 50mph legal maximum, because of the size difference.
The Space saver should be fitted to the rear axle only, regardless of where the puncture is.. so if you get a front puncture, you should move a rear wheel onto the front axle and put the spare on the rear axle. I see hundreds of cars tooling about over 50pmh with the spare on the front. You'd have to be a knob, frankly...
The space-saver tyre also has a poor profile and not very good wear or grip characteristics, so if you're going to be stoopid with it, make sure you have a ready-made excuse for the plod/insurance company.
Ralf S.