In addition to npadmani's recommendations, which I would follow if you do an actual upgrade, I would consider splitting your environment out and placing SRM on separate VM's. I would also consider some of the changes in 6 from 5.5, i.e., the ability to have external PSC's and the feature parody of the VCSA as opposed to the traditional vCenter server with a SQL database. The move to 6 is a perfect opportunity to reevaluate how your infrastructure is laid out.
Assuming you just to the in-place upgrade, you will also need to update SRM immediately following the vCenter upgrade, as the 5.5 and 5.8 versions of SRM are not compatible with vCenter 6.
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