From a purely academic standpoint, I was wondering if connecting LIPOs of different voltages in parallel (not that I would ever want to do such a thing with any type of bettery !) would likely result in a fire ? A fellow club member and I were discussing this, and he seemed to think that would only result in an imbalance condition - but the internal resistance of the batteries would have to be high enough to prevent an overcurrent condition I would think. Has anyone here ever known anyone to ever attempt such a thing and whether or not there was a fire ?

That is a big no no. No don't do it. No it won't be pleasant if you try.
The lower voltage pack is going to be hammered by the higher one as they try and balance out voltages between them.

You can do it in a series connection thou, it's done all the time.