SameerH, I'm a little concerned that you don't think that the problem is directly related to the eReaders firmware. The ePubs that are not working on peoples Kobo work everywhere else. So there is really no problem with the ePub file itself.
The ePub files are also using em as the font-size unit which is a unit that sizes the fonts relative to each other. So the ePub files do not have a fixed absolute font size set.
Also I did some research and it looks to me that the eReader firmware is applying styles to the <P> tags in ePub, I think this is how it is controlling the font-size and font-family. The reason I think this is the case is when I edit my troublesome ePubs and place the font-size: 1em; line on a class used for a <SPAN> tag within the <P> tags then the scaling works just fine.
If your not planning on changing the firmware to fix this issue, and you just planning to show people how to get around it, then your going to have a lot of upset customers. There is no reason why I would have to edit all my ePubs to work on my Kobo when there is nothing wrong with them when I use ADE, SONY PRS-300 or Calibre Ebook Viewer.