downconvert your HD footage to SD and work on a SD timeline so that all the footage will have the same format.
And if you are have the online process then, at the end of the edit, generate an edl- i guess here your work should end.
Now while doing the online confirmation, conformist will reconnect 16mm footage to your 16mm SD footage from tape and HD footage from XDCAM to downconverted (HD to SD) SD footage.