Are the locking nuts compatible with the standard nuts in the same set?

Yes. The 4 locks are M12 x 1.5 threaded and match the same extended profile as the 16 standard nuts, so you can distribute them evenly across your wheels for security without fitment issues.

Do these work with alloy and steel wheels?

These will thread onto any wheel with M12 x 1.5 lug holes. Compatibility depends on your wheel's center bore and offset — test-fit on one wheel before committing to all four to confirm the extended length works with your specific setup.

Can the spikes be removed if I change my mind later?

The spikes are a fixed part of the nut head and are not designed to be removed. If you need a cleaner look down the road, you'd need a different set.