From one of your links:
"both fire bellied newts and fire bellied toads can secrete skin toxins. It is thought that the toxins are different and distinct and they could potentially affect each other."
Note that it says they CAN, not that they do constantly. It also explicitly states that it is BELIEVED that they COULD, not that it has even proven that if they were constantly leaking toxins that they WOULD affect eachother.
In my experience I have seen nothing that suggests:
1-that they constantly leak these toxins. The toxins are an adaptation as a defense. They spend energy on them. They do not constantly leak them. If this was the case I find it hard to believe there are no observable effects at all. The guppies are healthy and breeding like crazy. Fish are highly sensitive to anything in the water and would in all likelihood have some sort of negative response to toxins in the water. The snails are also doing very well.
2-that they are toxic to eachother. If the toxin from the newts was toxic to the toads wouldn't the toads die or show some sort of negative effects after ripping the limb off of a newt? That physical attack is exactly what these toxins are there to prevent, yet they seem to be having no effect on each other.
Yes, many articles cite reasons for not mixing them. They cite differing parameters, physical attacks, and the toxin issue. Yet I have not seen any evidence to support this nor have I come across or been shown ANY scientific article showing that they constantly leak these toxins. The physical attacks are definitely a threat. But these are usually a result from under-sized and improperly setup enclosures, overcrowding, and underfeeding. The same results would occur in a community aquarium, so I guess we need to keep every species of fish alone too.
This setup is young. Many years ago I had this same combination setup for years without any issues the entire time. I really cannot take the word of others regurgitating what they heard from someone else about something they have not tried over my own observations. I have not observed anything that suggests they are inherently incompatible.
I am in no way saying 'they are fine together', 'go ahead and mix them', etc. I am saying that when their enclosure is properly setup, the populations are low, they are kept well fed, etc. it can work.