Imagine if caveman Slappy and Primitive Sponge met each over?
It would probably go the same way as it did in Squidferatu. They'd be vibing together on a singular braincell level.
The only downside is Slappy is most definitely cannibalistic and it wont be long until he tries to eat his new friend. I doubt this would be much of a problem because spongebob is a sea sponge. They can regrow their lost parts pretty easily. Sponge can take off his arm to feed his new friend and his arm will quickly grow back. Slappy gets an unlimited supply of food from his new pal :)
But there is the issue of primitive sponge bring easily provoked. I know for certain that cave-slap will try to provoke him for shits and giggles (because he's a little shit like that)
I can imagine they'd resolve their conflict easily simply because they're both too stupid to remember why they were mad in the first place and they both get easily distracted. They bond over their sense of humor and sheer empty headed activity.