What is the minimum distance allowed between two 802.15.4 devices in
order to prevent any RF (near field) or collision detection
circuitry/algorithm problems?

I can't tell you the minimum distance: all *I* can do is provide the
following quotation on EXPECTED range:

Depending on the RF environment and the power output
consumption required for a given application, IEEE 802.15.4
compliant wireless devices are expected to transmit in a
range of 10?75 meters.

Thanks, but I need the minimum distance, not the expected range.