If one of the restriction sites listed for my cDNA clone is "SalI/XhoI", can I just pick one of those to do my digest with?
No. Neither enzyme will cut. The cloning method used to make clones with this SalI/XhoI site creates a hybrid restriction site. It is half SalI and half XhoI and neither enzyme will work. You will have to choose an alternate enzyme based on the multiple cloning site of the vector.