I don't have any figures for the bandwidth required to maintain active sessions, but a helpful guide for the bandwidth requirements to synchronize session setup is:
roughly 500kbps for every 1,000 sessions set up per second
This is not an exact, officially published benchmark - only a rough guide from some internal testing. Session setup is far more important than active sessions.
As a best practice, you should consider using redundant HA links, as well as segmenting session-sync traffic from HA heartbeat messages. It's a high cost, but it pays off in spades: two interfaces for HA (use ones you don't really want, like 'HA' ports, or unused Fast Ethernet or Gigabit, where they are an order of magnitude smaller than other production ports on the device), plus one for session-sync traffic.
You can use non-accelerated ports or not as your preferences go - there's really no restriction on which port you use, so long as it's not already dedicated to another purpose. You could even theoretically use a production port shared with other traffic, though I wouldn't recommend this anywhere.