This is getting monotonous..... Ok, some things we've went over in the past.
1) We all (I believe) agreed at one time that if the pH is w/in the rhelm of almost as low as 6 up to 8.5+or so....most that I believe to be experienced here....we all agreed that you shouldn't bother messing with the pH.
2) The general hardness of the water more or less can be somewhat compared to having electrolytes in the human body. Some things work better or the body might be used to one mineral in the water. Either way there are substitutions for things.
3) Most lfs I've seen here and everywhere else have everything they own/sell on the same system, which means that they all have the same hardness, pH, temp (unless chilled comet tank) etc.
Honestly I think this is more of an opinion than anything. Maybe some slightly better accomodating conditions tailored to the fishes origin may slightly extend its life but probably not significant....$hit happens right, so pick your battles. Peacock kinda bluntly laid it out that I can probably guess that most here still don't have a perfect understanding of pH and GH from seeing some of the old threads. Rephrasing/restating, the water conditions can easily suit both make that a moot point, reason being that this is more of an opinion and not a factual debate.
Maybe I'm wrong but I've had mixed tanks practically since I started 6 yrs back in some way shape or form. Not boasting about my seemingly insignificant experience of 6 yrs compared to some of the other members buttttt..... I've never had a problem and there are definately tanks doing well that are mixed. All an opinion I say. Can we not call it a debate anymore?? All I'm seeing is opinions....I can visualize steam coming out of Peacocks ears just because he wants to spew some facts....please do!! That would make this a debate.