There are plenty of reasons why mixing is a very bad idea. Toxins is only one of the issues. Patogens, agression, differences in housing requirements, etc....are all good reasons to avoid it.
Specially agression in this case, i think is important. Bombina will jump and try to it anything that moves, and if a newt was to venture on land, it will sure get a bite (or get eaten if small enough), which will then cause stress to the newt, or even small wounds that might get infected.