First, keeping monitors(any animal) is betting against the house, that is, the result will always be the same, the animal will die, period. The quality of the bet is what happens inbetween. The quality of the task is what happens inbetween. This really needs to be understood by the newbies and the folks giving advice.
The question about keeping different species together, is not can they be kept together, but more like, does keeping them together increase the risks to an unacceptable level. This can only be addressed by the person who is keeping them.
We should all understand, there is danger is almost everything we do. We must also understand, that all these monitors will die. Some sooner then others. Again, what happens while they are alive is what is important. There is no arguing that.
In reality, the safest way to keep monitors is by themselves. Groups, any groups increases the danger of something delerious happening. It really does not matter if they are the same species or not. Monitors can and do, kill/comsume, members of their own specie, as well as members of other species.
This canniballism can be controled with knowledge of behavior and knowledge of husbandry. But it can never be eliminated, not with any type groups.
The answer to this question is like so many answers to so many questions, there is no hard fast answer.
Another reality is, if a beginer has to ask whether its OK or not, then most likely they do not have the experience to keep any monitors in groups, muchless different species. They should start simple and progress thru levels of difficulty with time and experience. That is only common sense.
The harsh reality is, and its straitup hardcase numbers(history), most of the newbies monitors will die quickly. No matter if they keep them in groups of same species or different species. Or kept singly.
I guess the only suggestion I can give is, maybe they should wait until they have the experience to not have to ask whether they should, because, surely they can. They just need to know how and why. Cheers FR

