I have setup few quota profile (15 GB, 20 GB .... 100 GB). All of our mysites have it's own content databases. Someone raised a concern and said MySite can NOT be over 50 GB. Is there any truth to this beside good practice?
The only limits on a My Site are the limits imposed by SharePoint (and they aren't unique to the My Sites). There are no separate limits for My Sites specifically unless you set them.
For 2013 see: http://technet.microsoft.com/en-us/library/cc262787.aspx