Those are completely different characters, not relevant to the Immortal Hulk storyline - except they could show up as enemies, but afaik there are no Rulks at the moment. Basically, they're completely different dudes that happen to turn into a red Hulk.
Basically: Bruce Banner has been heavily traumatized as a child. He was abused by his father and eventually his father killed his mother right in front of him. And subsequently threatened him to make sure he wouldn't tell anyone what dad did.
His father did all of this because he was convinced Bruce was a monster. Because of his work he was frequently exposed to low levels of gamma radiation and he got it into his head that his unborn child would probably be one of them mutants. Bruce wasn't a mutant, but he did happen to be an incredibly gifted child - but his father interpreted this as evidence that Bruce was indeed a mutant, a monster.
Due to all the trauma Bruce developed MPD, nowadays called DID - dissociative identity disorder. The childlike savage Hulk, with his basic speech patterns and always wanting to be left alone represents Bruce as a child. Grey Hulk/Joe Fixit represents his dark emotional side - things Bruce considered 'bad' because he saw how his father acted. Lust, greed, vindictiveness - stuff like that (hence Joe in Immortal Hulk talking like a gangster, hustling money, hitting on women).
When the gamma bomb went off and the Hulk was created, it didn't really create him. It just gave physical form to already existing alternate personalities in his mind.
Savage Hulk, Joe Fixit, and Bruce himself are the three main personalities - the three basics. Put together they should form a normal person, in theory. Joe is the id, savage Hulk the superego, and Bruce is the ego. That's why I also listed Bruce in my post, he's also a Hulk - i.e. one of the many manifestations of the personas residing in his traumatized mind (as an aside, it's debatable whether human Bruce is even his 'normal form' )
All of this was set up by writer Bill Mantlo (maybe with help from Barry Windsor-Smith) in the eighties. Writer Peter David took this set-up and ran with it - it forms the main theme of his entire epic 12-year run, so read that if you're intrigued.
Aaanyway, aside from these three core personalities there's a number of variations. If you've been reading Immortal Hulk, you know that Devil Hulk is a personality that Bruce kept locked up and hidden away deep inside his mind. He was fearful of him. Reread the conclusion of Hulk in Hell with all of the above in mind, specifically the conversation between Devil Hulk and Bruce.
Bruce used to turn into savage Hulk when he got angry or scared. When the gray Hulk was dominant, he used to change at night because Joe represented things Bruce was ashamed of - that couldn't stand the light of day. Devil Hulk is also an aspect of himself that Bruce wants to keep locked away, hence Devil Hulk also appearing only at night. BUT the twist is that whenever Devil Hulk loses it completely, his rage or fear gets the better of him, he reverts to Savage Hulk. That happened in hell and against the Abomination.
Devil Hulk is by far one of the most powerful Hulk personas. Thor said his strength was more akin to a god instead of a mortal. And like all the other Hulks except Bruce, he gets stronger as he gets angrier. But if he gets too angry, savage Hulk takes over. There's no limit to savage Hulk's anger so in theory there should be no limit to his strength as long as you make him angry enough.
Other important personalities:
Merged Hulk/Professor Hulk. Literally a merger of the three main personalities. The smarts of Bruce, strength of savage, the mean streak of Joe. Later reconnected as being a separate personality after all. Considered one of the 'heroic' personalities.
Green Scar: have you read Planet Hulk and World War Hulk? THAT is Green Scar. The most powerful of all the incarnations, no limit to his anger or strength, once caused an earthquake all along the east coast of the USA by taking a single step (they killed his wife and child, so he was REALLY angry). He's the one that earned the title World Breaker (hey now you know why it keeps getting referenced in Immortal Hulk!). Also considered a heroic personality.
Now note, as one of the side characters in Immortal Hulk has done a few issues ago, that precisely the heroic personalities are dormant and nowhere to be seen. Bruce and Devil Hulk have an understanding (hence Devil Hulk now being able to resist sunlight), savage Hulk is just here to smash and wants to be left alone, and Joe probably thinks it's hilarious if the world burns. But are all the personalities truly onboard with going to war against humanity? Or what Devil Hulk happens to consider 'wrong'? By any means?
Edit: man, I'm such a nerd. 42 year old guy here and this character still gets to me like no other All written from memory without looking anything up, this is just knowledge occupying space in my brain. Sorry for the long post, but this is the information about what the Hulk is exactly, that I feel is most relevant to what Ewing is writing at the moment. I think with this you should be able to pick up on more stuff and probably enjoy it even more! I hope.