Should I introduce my old crab to a new one?

[Social hermit]

It is almost March now and I am giving an update. I had decided to keep Papillion separate from the two new crabs, in a different habitat. Periodically I would put all three of them in a smaller 3rd habitat in front of me while I played video games, and they all got along pretty good, with plenty of play as well as autonomy. I had finally decided to put all three of them together the other day, in the same terrarium, but I observed Papillion going after the shy one (crawling on top, being persistent) and decided not to take the chance after all. I will continue to keep them separate but bring them together only under supervision. I have a feeling that the two new ones are older and can't keep up with Papillion's boundless energy.

[Social hermit]

Ok, last update. After a while I had decided to let one of the new crabs stay with Papillon unsupervised. The other one had started molting in a separate tank. And I went to work. I found Papillon dominating it, being on top. I used to think this was cute, but it became apparent to me that the newer crab was oppressed.

I separated them again. It seemed the newer crab was out of energy, lethargic, possibly paralyzed or in shock. I put it in the terrarium and put some dirt over it to let it molt. After 2-3 months, I checked on it today and found it dead. There are several things I could say about what I did wrong, in hindsight, but my main conclusion out of all of this is:

If you want multiple hermit crabs, then get them all at the same time. Don't add more later, it's very dicey. If you do, then it would be best to keep them separate. Note: I confess, I did not try the suggestion of putting them in the common water area.

On the bright side (the silver lining), the other hermit crab that was molting survived the initial molt. It came up to fill up with more water, and went back under for phase 2.

* FYI don't bury a hermit crab, as if to prompt it to molt.
For one thing it needs to fill up with water before doing so.

[curlysister]

Many people successfully introduce new crabs. A quick dip in the fresh water (for both old and new) ensures that they all smell the same and that nobody is seen as an intruder.
Crabs make tunnels and caves underground when they molt, which is why it is important to have substrate that is sand castle consistency. If tunnels collapse, or if we cover them so they don't have a nice little 'cave' under there, they will suffocate.