How safe is it for older people to self medicate with Paracetamol Mylanta and Diclofenac?

In terms of the interaction between the two, according to the British National Formulary (the big book which doctors consult - you can access this for free if you register online) it should be fine. Providing you follow the instructions on both.

But to be honest, if you're suffering enough pain that you feel you need to take both you really need to be going to your doctor so they can possibly prescribe you something stronger.

On another point, diclofenac is prescription only anyway so you should really only be taking it when absolutely necessary and for the thing you were prescribed it for. As it masks the pain, if it's for a muscular injury, when you have already taken it it's quite possible to further damage yourself and not realise because the diclofenac masks the pain.