Tom,
We have discussed with you the fact that the kribs are improperly homed in the 70L, if you were going to rehome ANY fish, they would be the best choice. Rummys are fine in a 70L, and you could even get a few more, were you to rehome the kribs.
As for adding smaller fish to the tank to go with the kribs, that's just a bad idea. Smaller fish would be targets for the kribs much more so than the rummys.
Regarding the betta with kribs... as Wildbetta pointed out, they are both rather aggressive and since the kribs have ALREADY gotten aggressive in the past, it would just be asking for trouble.
Please take the advice we are giving and either rehome the kribs, or leave things as they are. Switching things around will just be a potential trigger for trouble.