I think it would depend on a number of things (speaking as a cat owner, rescue dog owner and rescuer). :)
Whether the possible financial expense of owning an elderly cat would be a strain on your budget if you had a dog to insure, feed, provide for, vaccinate, find holiday care etc for too when, heaven forbid, the cat might become poorly as she ages.
Whether you think that Mog would be scared or intimidated by a dog, no matter how calm. Look at past history - for example, does she leg it when your 5 yos friends come to play or is she content amidst the added creatures and noise?
Can you provide her with somewhere to escape to if, for example, she is poorly or if she becomes more nervous or withdrawn as she ages?
I'm sure others will think of more, I'm brain dead and knackered this evening!
But, if you think she and you can cope, then as you say it is perfectly possible. Please ensure that you find a rescue that does all the stuff I bang on about in virtually every thread, including a home check as this in itself will help rescue consider your cat's needs. An older dog who has a history of living with a cat and who has been cat tested/fostered with cats too would be your best bet.
However, bear in mind that even a dog used to living with HIS cat may initially decide to have a fake chase of one in his new home although that is highly unlikely to last. Of course the other thing is never to leave the two unattended in the early stages at all and even when the dog is an established resident it may well be unwise to leave them together whilst you are out.
Also, always make sure that the cat has an escape route. A friend leaves her dog in the kitchen when she goes out (not often, she's a SAHM), with the door open but a stairgate in place so mog can get out if he is fed up with the dog.