Unfortunately, for the piranhas you are dead on. Gratz to you though jonny_dw for doing your homework and not impulse-buying a piranha.
Other than geryi, and even then that's for experts only and still rarely works out, you never want to have more than 1 Serrasalmus per aquarium.
I wish I could help you out on where to get it in the UK but I only know how to get them in the US. Not sure if it means anything but in the US sanchezi are relatively easy to get so hopefully that means it's not too hard over there.
Actually, some people have kept serras besides geryis together. I know frank at opefe did a cohab project with some P. nattereri and cariba, one S. maculatus, and 4 S. sanchezi (didn't last forever though and he experienced a few losses). Another person on a different forum bred gold spilos and a few people on that forum have some cohab projects going.
I agree that some have tried it but 99% of the time it doesn't work at all or not for long and that's of the advanced aquarists.
Just as some know that have had rbp's, if you feed them minnows or add other fish the fish could last for days months or even a year but chances are almost 100% that those fish will be killed sooner or later. I wouldn't consider that cohabitating or successful.
Frank himself doesn't sound too sure of the success...
If you are a frequent visitor or member of a piranha forum on the internet, you have probably heard it before i.e., my piranhas are cohabitating together in a fish aquarium. Are they really? What is the definition of cohabitation? And how does it apply with piranhas?
There will always be an elite few that will try disprove the nature of the beast only to find out the beast thanks them for the meal.
I think it CAN be done if enough people try it as perhaps S. rhombeus may also be bred in an aquarium hobbyist setting someday but like I said best left to the knowledgable ones and not the beginners.
I am not trying to disagree with you because I do agree with what you said and realize I'm nitpicking so I hope you don't take it that way or think I'm arguing with you.