I really don't think it is advisable to use different guns with pour-ons. Each product has specific nozzles and instructions for specifically that product, and they are very different. The Crovect gun has three ways to be used, depending on whether you are protecting against or treating flystrike, or protecting against ticks & lice. Vetrazin is used differently and has a different applicator again.
thenovice, I would say that your supplier wants a serious ticking off - they should know that Crovect needs its own specific nozzles. Fitting the top of the container is simply not good enough.
These products are extremely noxious to the sheep, to you, to your shearer(s) and to the environment, and none of them are cheap. It is surely worth getting the correct applicator to ensure that you are applying it correctly.