Ok I know it's a bad idea but if in a bind what would happen if you tried for example in an E8 buggy to run a 5600 40c and a 7200 80c together?

Does it just restrict down to the lower cell and is balance quickly or does it literally destroy the lower spec cell/risk fire etc?

Just like any lipo your pack is only as good as it's weakest cell. I have seen a couple guys run completely different packs with no problems at all. I myself have never tried it. The only issue would be run time. You wouldn't want to run your 7200 lower to where it would effect the 5400. Best way I guess is run it for a few minutes and check battery on the charger to see how much mah it used.