Cat foaming, shaking, sneezing after flea/dewormer. Is it serious?

Please have her assessed urgently by an emergency vet and take the shampoo bottle with you. Small kittens are very sensitive to insecticides and weakness and tremors are common side effects and can progress to seizures

I'm sorry to hear about Nylon. If he was just recently treated and is now having a severe reaction to the treatment it is best to contact the vet that treated him and bring him back in for care immediately. They may need to place him on IV fluids or other measures to reverse any reactions and get him recovering. If your current vet is not available, seeking out an emergency vet in your area is best.

Panting is usually a sign of serious respiratory distress in cats and I would not expect this as a side effect of nitenpyram or piperazine. I would recommend taking Pudge to an emergency veterinarian if you continue to see him panting. If he has fleas it's not uncommon for them to cause discomfort or increased itchiness as they start to die so I wouldn't be concerned about scratching. Nitenpyram only lasts about 24 hours so you should follow up with a longer term flea preventative.

You did exactly the right thing by washing off Odin and Toby right away. Continue to monitor them, and if you see any additional signs of toxicity such as muscle tremors, vomiting, diarrhea, depression and incoordination, I would take them into an ER vet right away. Otherwise, I would have them both examined by a vet in the morning to be safe.

Cap star is safe to use with other flea treatments and generally any topical can be applied 2 weeks after another with no problems. It is not uncommon for a change in stool after a deworming. If they are not normal after 24 hours from medicines then I would worry and have them checked

If your cats are showing signs of poisoning, this is a medical emergency and they need veterinary treatment now. There is nothing you can do at home for them, as cats are very sensitive to flea treatments and any time they show signs of poisoning, veterinary treatment should be sought right away. Please take them into an ER vet clinic; if there isn't one in your area, there will be a vet on call for emergencies. The information to reach that vet is found on a clinic's answering machine or website.

I recommend to have Grover examined by the veterinarian right away. Pets can have seizures for many reasons including poisoning, a reaction to the flea medication, liver disease, electrolyte imbalance, infections, inner ear problems or epilepsy. He will need a physical exam and bloodwork to begin to narrow down and appropriately address the cause. Seizures are serious and are best addressed early. If it is safe to do so, you may wash Grover in dawn dish soap to remove the flea product but he still needs to be examined. Make sure to bring in the package of the flea product. I hope this helps and please feel free to post any additional questions.