Can these be used together?
You could use it together but the resultant speed will be horrible. As for any gotchas, I am not really aware of any.
I highly recommend two-part article by @blacklight447 (who is a member of PTIO team):
Part 2, in particular explores usage of Tor + VPN (but it is better to read both parts, to understand what you are doing and why).
In short, article argues: you could use them together, but should not.