Symbols are units of data (just like bytes are units of data for MM). Here is some terminology:
Symbol --> unit of data
ST data width --> total width of the data port (containing one or more symbols per beat)
Beat --> transfer of data from ST source to ST sink
So if you had a 32-bit ST port containing two 16-bit symbols you would declare the ST port as having two symbols per beat.
A packet is typically formed of one or more beats and not just a really wide ST source/sink. As you send the packet you signal the start of the packet by asserting the start of packet signal and on the last beat you assert the end of packet signal. So unless I'm missing something here I think what you want to do is expose a streaming port and send one or more symbols per beat to the exported port.