So, I'm just wondering what things are needed to be done to have the Powder Gangers be a successful gang during the Ending slides! I want them to grow big and strong! Things I've done so far:
1.) I helped Ringo (or whatever his name was) take over Goodsprings. No Powder Gangers died during that battle.
2.) I helped the Powder Gangers during the suprise attack at NCRCF, and only about two Powder Gangers died.
3.) I helped the Powder Gangers in the Vault a little bit, and then I got them to join the Great Khans.

Is there anything else I can do to help the Powder Gangers out? And can you tell me which main faction to join by the end of the game that will help out the Powder Gangers the most?

Thanks for reading and suggesting anything!

Rescue them from the Legion camp?
Other than that, don't support the Legion. Ideally you should support Indy, though House and NCR could also work; don't remember, but I believe they ignore/fail to overtake the prison.

You should support Indy or House. Both NCR and Legion will crush them if you go with their endings.

Doesn't NCR only attempt to, but fail miserably?
But yeah, even if it's just an attempt, it's still worse than being left alone, obviously.

Unless you assault both the Vault 19 Powder Gang and the Correctional Facility and kill their leaders, going independent will allow the gangs to do more or less as they please, and the ending slide says they terrorize for years. You don't even have to help them.

Nah they defeat them quite easily:

"With the Dam firmly in their grasp, the NCR turned its attention towards wresting the Correctional Facility from Powder Ganger hands. The Powder Gangers are no match for the battle-hardened troops of the NCR, and summary execution awaited the Powder Gangers who managed to survive."

You're thinking of the Fiends if the player doesn't weaken them maybe?