You should be able to mix and match within a pair.
People get confused on this. RDRAM always has to be in pairs. The confusion sets in on the 840 and 850 chipsets. These chipsets actually require 2 pairs of chips (i.e. all 4 slots full) since the chipsets have 2 RDRAM channels. For the 840 and 850, the 2 pairs must match (i.e. if one pair is a 128 and a 64, the other must also be a 128 and a 64).
When making pairs of RDRAM RIMMs you can substitute a CRIMM (continuity RIMM) in place of an actual RDRAM RIMM to complete the pair.
So, within a pair, you can mix and match and should be able to have 1 ECC 1 non-ECC so long as you have ECC disabled in the BIOS. I am not positive on this, though. Also, you can have a 128 and a 64 as the pair.