Welcome to the Community -
If I have one vDS using 10GE uplinks, can I add the IBM (older) hosts that are using 1GB uplinks? - Yes you can just remember the uplinks will need access to the same vlans
Are there any considerations or risks in this setup? - You will need to make sure the ports are confiugred the same way in terms of vlans etc.
How will this affect functions such as vMotion, HA, etc? - Should have no impact