We have had hermit crabs for 10 years, but we have never introduced new crabs into our mix. My son got 2 crabs for his 6th bday from a relative, in one of those little plastic crab cages w/ a food shell and a sponge

Last year we had one die right after molting (dug between the water bowl and the glass and got stuck, I think because she was still weak from molting, I didn't find her in time to free her). Two months ago our largest (and my son's favorite) Elma Fudd was came up from a molt naked, leaving her shell behind in the hole. We think her tunnel must have been collapsed from another crab digging nearby and she came up for air, too weak to bring her shell. We isolated her and got her to re-shell, but she died a few weeks later.
Because we are now down to 3, we bought 3 more hermies to introduce. We got the largest we could find, but they are still very small compared to our 10 year old crabs! Our 3 big ones are larger than golf ball sized, the new ones are quarter sized and smaller. We tried to introduce after a week in iso and the bigger crabs seemed aggressive so we separated again. I waited until everyone had decompressed and tried again today (about a month later) bathing and introducing one big and one little who were up and about (and had been for a few days) in the big tank. The little stayed hidden, the big went straight over to investigate, climbed on top of little's shell and was hanging on. I let that go for a little while but then all of a sudden biggie quickly flips over little's shell and starts trying to grab inside. I separated and put little back in iso tank (biggie DID NOT want to let go, and is still very active/agitated walking and crawling all over tank).
Suggestions? Is it possible to introduce? What should I try?
Thanks!!