Dear Marion, we may not mix the use of two products Frontline and Profender. Of course, both can give an allergic reaction to a cat, but not necessarily always and certainly not to every cat. So far, I never had any negative experience with Profender and only one of my cats did react on Frontline (lots of salivation). Considering that it only concerns a few drops in their neck, even this cold will not be a problem (they certainly are not getting half wet or so). The fact that Mohrchen was sick of Frontline can indeed give an indication of an allergy, but it can also be that she was having there a little wound. And of course, it is always wise to ask your vet for advice in case of doubts. However, in stead of trying to give them pils, there is still another solution, namely the use of Catminth (Pfizer product): it is a kind of a pasta. I used to give it to very young baby cats at the age of 6 weeks straight in their mouth and for the older ones, I mixed the quantity in their food. But then again, it depends a bit on the cat: some of them just ate it and others gave me the message “enjoy it yourself”. It is also an easy product to use and the doses to give are clearly indicated. Therefore, if you would wish to try this, make sure to mix it in their most favorite food, mingled with some meatloaf or fish or chicken.
With regard to your question in connection with the neutering of the females: if I understood it well, you plan to neuter Emil next week. Well, if that is so and you know/hope that no other males are around, you do not have to worry about your females. You will be safe for another few months. But nevertheless, neutering a female cat at the age of 6 months is OK. I once was obliged to do it and this cat is still very playful and funny. And again, some of them were sick for a few days, and others did recover sooner (just like we humans, I suppose).
But in the end, I know for sure that your Spreewaldkatzen do have a great and good life with you and Peter to care for them. And we have the big fun of reading their and your stories and adventures.