I am sorry your kitty is having trouble. I do not think that worms are the culprit here. Cats are considered to be geriatric at 8 years old, and as they age become more prone to illness. The 3 most common ailments that afflict elderly cats are diabetes, hyperthyroidism and kidney disease. The symptoms include vomiting, diarrhea, increased water consumption, increased urination, poor hair coat, weakness, weight loss, muscle wasting, dehydration, changes in appetite, increased vocalizations and changes in behavior. The illness most commonly associated with with ravenous appetite and weight loss is Hyperthyroidism. Your kitty should see her vet for an exam, comprehensive blood and thyroid panel and urinalysis. Any illness will be diagnosed and can be managed. Some radiographs may also be indicated, depending on what the bloodwork reveals. Please see the following website for more info on hyperthyroidism.
As far as your other questions go, you can use over the counter dewormers, but be careful as some are toxic to cats. And, most of the time if you are seeing weight loss or severe symptoms, it is from a parasite load that is too heavy to be treated by anything but prescription medications. And even though the over the counter dewormers may be labeled as broad spectrum, they will not treat or eliminate some of the more dangerous parasites. Most intestinal parasites are not passed from cat to cat, but the dog could become infected if he/she tries to be helpful and cleans out the litterbox for you. If you are concerned about parasites, your vet can run a fecal panel at the same time the other diagnostics are run.
Please let me know if I can be of further assistance. If this has been helpful, please hit the green accept button. I hope all goes well with your babies!!!