I don't know any issues with vCO 5.5.2 and the vCAC 6.1 plug-in. I used the vCO 5.5.2 with the vCAC 6.1 plug-in on vCAC 6.0.1 without problems. On an other environment I've updated vCAC from 6.0 to 6.1 while during upgrade the vCAC plug-in was also updated which also works fine. So from my experience this all works fine.
You could test by deploying a new vCO 5.5.2 appliance (only vCO which should be done quickly), install the vCAC 6.1 plug-in, synchronize workflows and check everything works fine.