I use the 7/8 on my Helios 7 weight switch.
You might be getting different advice regarding lines from shops as they may be selling different styles of line.
For example, my 7 weight throws the following:
525 Grain Rio Skagit,
480 grain Rio Scandi
Mikes 7/8 Switch which is 425 grains
7 weight Hydros Bank Shot which is 270 grains and
9 weight Rio Outbound short which is 375grains.
It all depends on the profile and length of the line and the casting style.
Took me a wee while to get my head around it, but it does make sense in the end.