First, what would be a good title for a Pokemon/ Sailor Moon crossover with all original characters? So far, I call it PokeSenshi, but I need a better name.
There's two ways to go about this:
The first way is to do what magical girl anime series usually do and name it after the main character. So, for example, if your main character changes into Sailor Pikachu, as lame as that is, you could simply call it Bishoujo Senshi Sailor Pikachu. Yes, it's incredibly corny, but you're delving into shoujo anyway. That and with an entire cast of OCs, presumably in the Pokemon world (although I'm still trying to figure out what you're doing here), you'll want to make it clear that it is a Sailor Moon crossover and not just a Sailor Moon rip-off in the Pokemon world.
The second way is to take a major element or theme of a story and use it as your title. So, for example, let's say you're having the Moon Princess be reincarnated as a young girl in Hoenn who's setting off on her quest, only to realize she's destined to become a Sailor Soldier and fight evil. If you wanted to focus on her remembering her past self, you could have the story be called Voices of the Moon. Or something far better than that because I suck at titles.
In any case, we can't help you too much here because we don't know the story. However, the closest thing to a suggestion that I can personally give you is a set of possible paths you can take. Think it over.
Second, which forum would I post a Pokemon crossover fanfiction? In the Pokemon FanFiction or Other Writing?
Because it's a Pokemon crossover, you can still post it here, particularly if the Pokemon influence is strong. (If Pokemon is just a passing thought, you might be better off going in Other Writing.)
Third, what could I use for a title of a Pokemon/ Sailor Moon crossover that involves the girl gym leaders, Ash, May, and Team Rocket?
Professional ninja. May or may not actually be back. Here for the snark and banter at most.