When my mom's dog comes to visit there is always the risk that Filou will start to pee outside the litterbox although Amber is very good with kitties.

What you can do is: how many litterboxes do you have? Best is to have as many as you have cats.

Next: give the culprit much much attention.

Next: make sure he has a place of his own where he can hide. Even if Alki behaves very well, Felix may prefer to avoid him. Make sure he can reach the litterbox from there without interfering with the dogs.

Buy a blacklight and look where there is pee. You will have to clean everything spotless else Felix will try to go back again. Others here will know better what you can use for cleaning as the breands are different in Europe and in the US.

Good luck!