HI there -- Welcome to Just Answer.
Zeniquin is a ood broad spectrum antibiotic that would be given for skin or respiratory infection.
There would truly not be an improvement by adding clavamox or antirobe to the regimne.
Often upper respiratory infections are caused by viruses -- with antibiotics used to prevent or treat secondary bacterial infection.
Most respiratory infections pass in 7-14 days -- and the single biggest thing to watch and monitor is eating and drinking. Often a cat that is uanble to smell food will decrease or stop eating.
So be sure your cat is eating -- and if true nasal congestion is preventing this or affecting him -- then a call should be made to your vet. Your cat may be scheduled to come off the antibiotics before the respiratory infection is cleared -- so your vet may choose to change antibitotics or continue the Zeniquin longer.