The SB-900 doesn't have an overheating problem. The problem is User perception. Speedlights were never designed to be fired continuously as fast as they can recycle. Check the owners manual on any Speedlight. The significantly faster recycling time of the SB-900 combined with a machine gun shooting style can result in themal shut-down. The SB-800 has a recycle time almost twice as long depending on the type of batteries being used. They can be fired continuously until they melt. I have seen it happen. The thermal shutdown feature on the SB-900 prevents the flash from being pushed to distruction.
The current SB-910 extends the recycling time as the Speedlight heats up to prevent the thermal shutdown from kicking in.
Both the SB-900 and SB-910 zoom out to 200mm compared to the SB-700's maximum range of 120mm.
It you don't plan on using flash beyond 120mm the SB-700 or even a refurbished or used SB-600 (max. Zoom 85mm) will suite your requirements very well. More so since you are looking for the smallest and lightest footprint.
If you need the additioan range and power of the SB-900 using common sense goes a long way in preventing thermal shutdown. Good Luck and Enjoy your Nikons!