Some time ago I asked this. Potara fusion limit
In DBZ, we see Picollo fuse with Nail and exponentially increased his power. Then again he fused with Kami to further enhance this.
Was only Picollo/Kami able to do this type of fusion? I mean if you're on the verge of Genocide, you'd think some of the warriors may try fusing.
So was this due to knowledge (Nail new from the old Namek, Kami cos he divided his body) or physical limit of Namekians.
Associated, how many times could this be done?
Ankit is actually incorrect about this. The fusion between Kami and Piccolo was indeed special. Nail clarifies that Namekian fusion is only possible between two Warrior Namekians when he says he's the only Warrior Namekian on Namek apart from Piccolo.
Kami is of the Dragon Scion Namekians, the ones that can creat dragon balls and are generally weaker than their warrior type counterparts. The fusion between Kami and Piccolo is a special one because they were originally one Namekian. Thus allowing them to fuse.
The normal Namekian fusion can be done for as many Warrior Namekians there are. But the manga establishes that Nail was the only Warrior type on Namek. Probably because Guru worried about the preservation of his race so he mostly made Dragon types as they are the only type that can produce eggs.
Funnily enough the guidebooks go against the manga as there is more Warrior types than just Nail.
List of references:
Chapter: 285 (DBZ 91), P14.2
Context: talking to Freeza
Great Elder: “Nail won’t be as easy to defeat as you imagine…He’s the only Warrior-type Namekian on this planet. Things won’t go as they have with the Namekians you people have killed.”Chapter: 295 (DBZ 101), P1.4, P2.1-5, P3.1
Nail: “I-I’m astonished…I don’t know what kind of training you’ve done, but you’ve acquired unbelievable power…Still, it’s unfortunate…If you had only returned to the original, single Namekian you were, you might have been able to defeat even Freeza…”
Piccolo: “Are you saying that if I merged with God once again, my power would even surpass Freeza!?”
Nail: “Th-that’s right…I was overwhelmingly defeated by Freeza, but I should have a good comprehension of his abilities…[ ] So merge with me…! I’m also the only fighting-type Namekian on this planet…[ ] That’s right…Your power will become many times greater…”
Piccolo: “…You’re not lying, are you?”
Nail: “If you think so, you can just go get killed by Freeza…”
Translated Daizenshuu 4 excerpt
Incidentally the three "warriors" that attacked Freeza when he reached Guru's house was filler.
Was Picollo/Kami a special case?
No, Picollo didn't even know about the fusion and it was Nail who informed him how it works. That means it's limited to Namekians race and any two Namekian can do it.
How many times could this be done?
Nobody knows it happen twice in anime series and it's still running, so future might tell.
From dragonball.wikia.com
It is more of an assimilation (the alternate title of this type of fusion) than fusion, as one Namekian absorbs the energy, strength, memories, and intelligence or thought pattern of another. Both participants must willingly consent to fuse in this manner and usually the pair agrees to the stronger of the two to serve their body as the host; the reason for this consent is that the body of the absorbed Namekian disappears during this fusion and the consciousness ceases to exist as a body. The agreed host places his hand over the chest of the other one and through unknown thoughts, a flash merges them. After the fusion, the non-host's consciousness lives inside the body of the host from this point onwards and their life span as a body ends. The host's consciousness dominates the body, while the non-host serves to aid him by supplying information, thereby meaning that the absorbed consciousness remains separate from the host's.
The largest difference with this type is that once fused it cannot be undone even by methods that can undo Potara fusions (such as being absorbed inside Buu, evidenced by the fact that Piccolo was found in Super Buu's body, but not Nail or Kami). Both the host and his partner maintain separate consciousnesses: while the host has control of the body, the partner can watch from inside and is capable of speaking with the host's consent; during his fight with Imperfect Cell, Piccolo gives Kami credit for coming up with the idea of playing possum in order to extract information from Cell.
