I need just a bit more information to help you.
What breed and how old?
How much does she weigh?
How long has she been on the medications?'
When did the seizure occur and how long did it last? Is she back to normal now or still uncoordinated?
For her weight the vetmedin dose is on the high end, but not an overdose. Same thing with the lasix. High end of the dose, but not an overdose. The hydrocodone dose is fine. None of these drugs are associated with seizures or neurologic signs.
The seizures may be related to the heart disease if the brain is not getting enough oxygen, however it is more likely that they are a whole new problem. Seizures can be caused by "outside the brain" metabolic problems such as low blood sugar or low thyroid hormone. If your veterinarian did not do bloodwork when starting the medications then you should have it done to check for these things.
Most seizures are triggered from problems inside the brain and at her age a brain tumor is possible. These are very hard to diagnose. They require and MRI and exam with a neurologist. Your vet can prescrible antiseizure medications that may reduce the seizure activity, but will not provide a cure.
I know none of this is good news. Talk to your vet about reducing the heart medications doses IF she is not coughing or breathing hard. If she is coughing then don't decrease the dose. Do bloodwork if it is not done. And consider some antiseizure medications.
I hope that helps.
A mild anemia at her age may be related to any chronic/ongoing problem including the heart disease. It would have to be severe anemia to cause seizures. And yes, a tumor could be causing the seizures, but it would be an internal and probably a brain tumor, not one you see on the skin.
She is on such a high dose of furosemide that I am not sure the added diuretic effect of the garlic is helpful, but it doesn't hurt. You can get taurine and carnitine at a health food store and it will not do any harm.
And if she is seizuring that frequently I would call your vet and see if he will prescribe some phenobarbital. It is not likely to cure her, but if you can reduce the seizures you will improve her quality of life.