I'm not certain I understand the "single instance of vsphere 5.5" question. SRM requires two vcenter server instances and two SRM servers deployed (one at each site). If this is your case, please read below 
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The answer is Yes, this is possible. And Yes, there are caveats.
1. A single VM cannot be replicated with 2 different replication technologies and protected by SRM. This is not the case here, as far as I understand your situation.
2. SRM has different limits when dealing with array-based replication and VR. You can replicate and protect up to 500 VMs with VR.
3. There are certain limitations if you want to use a single console. You can manage both VR and SRM through the SRM Plug-in to the desktop client, however certain VR capabilities are not exposed in it (most notably multiple points in time). If you are OK with using the Web-client to configure VR and the desktop client to configure SRM, you will be able to leverage the best of both worlds.
Hope this helps.
