Is it possible to run different ros distributions and still communicate with each other in a distributed computing setup?

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Thanks! It's working for me now! Just that I had to export the ROS_IP in each of the computers.

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Just a note: this only works in case message definitions have not changed between ROS releases. For messages in the 'core' (ie: std_msgs, sensor_msgs and similar) that is often the case, but for others it may not be true. You'll get MD5 mismatch errors in case things are not compatible anymore.

What is the best way to check if this has happened? For example if I wanted a Indigo and Kinetic distro to talk to each other?

Not sure it's the best way, but rosmg md5 $pkg/$msg will calculate the md5 for you. You could then compare versions in different releases.

Note that you'll automatically get warnings about mismatched MD5s when trying to setup comms though. It's not like things silently fail or ignore it.

So an example would be rosmsg md5 $geometry_msgs/Twist? I'm obviously not using the correct syntax because I get invalid type error messages

No. Use this:

rosmsg md5 geometry_msgs/Twist