You only need to do one thing here, and that is to find another garage, i wouldn't have fitted the shoes unless you had authorised the wheel cylinder(s) at the same time.

You really shouldn't be driving this car at all till its fixed properly, complete brake failure is a possibility if that weeping seal in the leaking cylinder gives out completely, anyone's guess what the rest of the system is like, please get it checked over completely by someone competent.

At the very least before you drive it again check the brake fluid revervoir is topped up, and whilst parked stationary at home stand on your brake pedal as if doing an emergency stop, if the seal or some other weak part of the system is going to blow, better it does so on your drive than during a panic stop on the road for a real reason, you could end up with the book being thrown at you if something tragic happened.

Are the brake cylinders supposed to be changed when the brake shoes are renewed or only when there is a suspected leak?

Not changed unless necessary, but if you in glancing saw the cylinder was leaking then a mechanic should have seen flashing lights in front of his eyes and heard alarm bells in his head.

The car is now older, i would have expected to find at least one leaking slave cyl behind those drums had i been doing the job, pity it has to come apart again it would have been so much simpler to fit the new cylinders wth the brakes already dismantled...bear in mind the rear brake pipes may not come undone without twisting up when the cyls are changed so expect a few more ££ for new pipes if that happens.

Don't know about Focus but rear brake cylinders on Escorts were designed to be SLIGHTLY damp around the seal area.