From what I have gathered: some people keep multiple species together with no problem. However, some have found to have problems when mixing species so it's quite a debatable topic. This is just some info that I have collected, but as I stated, it is a debatable topic so this isn't really a concrete list.
It's recommended not to keep Indo's and Viola's together. Indo's are shallow molters and violas are hunters so they have been known to dig up the indo's.
I know Straws, Equadorians/compressus or indos can mix with pps. Es also prefer higher humidity 80%. Straws are the hardest to keep. They require 80-85F and 80-85% humidity no fluctuation. If you mix them it's recommended to get the same size straw or a bit larger than the crabs you currently have.
Es/compressus also need a higher amount of calcium in their diet. This species has the most trouble molting and has been found to get trapped in their exo during the molting process.
C. Purpureus/Blueberry hermit crabs have a bit of a debate about them, some say they are too shy and will not stick up for themselves so they should be housed alone. Others have noted aggression around food and believe they can be housed with mixed species. needs 80F/80%humidity
Ruggies I've heard get along with pps but I've also heard that they have been noted to be pretty aggressive so it's more recommended to mix them with Es. Some recommend keeping them in a tank by themselves. I've also heard that they can be pretty docile and warm up to people quicker than most species. So as you can see there's a bunch of mixed info. needs 80F/80%humidity
Acknowledgements and References:
I would like to thank the HCA for putting all of the care and work into hermit crabs and creating an environment for members to share experiences and learn.
Also, thanks to the rest of the hermit crab community for all the hard work and dedication put into studying these fascinating creatures if not for them we wouldn't have such informative sources to learn from.
never seen any other kind of aggression. the crabs getting stuck in exo makes so much sense though, i've had that happen a few times. come across them during a deep clean, help them out of the exo and sometimes the exo is so narsty by that point you have to get rid of it, but honey & cuddlebone fix that 
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