If you want to be able to use the vSphere-client without problems dont use VMDKs larger than 2040 Gb.
You can use VMDKs upto 64 Tb if you dont mind using the Webclient.
Yes you can add another disk of 500GB. In fact you can have up to 60 virtual disks attached to a vSphere 5.5 VM
Please find the configuration limits here for what you can and cannot have: http://www.vmware.com/pdf/vsphere5/r55/vsphere-55-configuration-maximums.pdf