Your problem may be caused by fast acclimatisation, especially if the fish came from different water hardness parameters. The method you use is only really suitable for moving the fish between two tanks of the same(ish) water parameters because the changes between the parameters are too fast for the fish to adjust. I attempted to write about why it is important to acclimatise at http
/aquariumadventure.wordpress.com/2011/10/07/importance-of-acclimatisation/ and what I consider to be the safest method at http
/aquariumadventure.wordpress.com/2011/10/13/methods-of-acclimatisation/
I recommend against guppies, the ram and neons may have a go at them.
As for "an expert", how do you really know that they were or were not? It might be a better idea to take the average of the advice from a few sources, definitely ask the people, but also do look at written sources too!
What reasoning did they give? My recommendation is based on that I have only ever seen 5-6" Bolivian rams in large tanks, while in small tanks I have only ever seen smaller adult specimens, my observations of various South American cichlids (many of which appear to benefit from interaction with their own kind) and my experience of seeing various sized fish in various sized tanks (I like to see my fish have enough space to swim, as opposed to just turn around).
Edit: thanks minnnt