...honestly we play pretty fast and loose with mixing alloys around here. We really shouldn't since it can be very bad, as you know.
The three RoHS alloys I have experience with are SAC305 (blech, no idea why people tolerate this stuff), SN100C (seems to be great on new assemblies, never much tried mixing it), and REL22 from AIM (this is on our "high strength" spool holder, next to the Sn62Pb36Ag2 silver solder, so people tend to graduate toward it for no better reason) which seems to do pretty well in typical lab conditions after being used to rework either SAC305 or SnPb joints. We don't do real testing around here.
When I am knowingly mixing alloys I will try to clear or flood the joint or both (remove as much old solder as possible with braid, or add new solder in excess). We are pretty good around here about using Way Too Much flux, which is of course the correct amount. This might help. Either way we seem to do okay for this light use.
The three RoHS alloys I have experience with are SAC305 (blech, no idea why people tolerate this stuff), SN100C (seems to be great on new assemblies, never much tried mixing it), and REL22 from AIM (this is on our "high strength" spool holder, next to the Sn62Pb36Ag2 silver solder, so people tend to graduate toward it for no better reason) which seems to do pretty well in typical lab conditions after being used to rework either SAC305 or SnPb joints. We don't do real testing around here.
When I am knowingly mixing alloys I will try to clear or flood the joint or both (remove as much old solder as possible with braid, or add new solder in excess). We are pretty good around here about using Way Too Much flux, which is of course the correct amount. This might help. Either way we seem to do okay for this light use.