Never done a "Multiple Player Races on a single planet" campaign, so I can´t help you here.
All I can suggest is for you to read some of the fiction around here (which you probably allready have)
Steves Trans Newtonian Campaign
viewforum.php?f=16Kurts Six Power Camaign
viewforum.php?f=42I belive you can mitigate the low tech aspect of your conventional empire by setting up the game with rather low populations, and after game creation, increasing population and industry in SM mode.
The size of pre-build NPRs and the NPRs activated by NPR activities seem to be based on the player population at game creation (not sure where I read that, Kurt perhaps?).
So, if you start with 500 mio people and the default industry for that size of population, and after game creation change this to 1 bio people and double the industry, you give yourself a boost in the "catch up game"
Edit: Thinking of this, using several player races earth should do the trick too (in some way). If the NPRs size is based on the largest player race, but you have several player races, the combined population/industrial power would be much larger. Of course, working together is probably not what you want all those power blocks on earth to do