Let's talk about the first Devil Fruit part:
This can be the stats cap for this. Depend on devil fruit you can choose which attack has basic attack power which one has supreme attack power
1-20 Basic Mastery [The attacks that come in the mastery of this will probably be D ranks attacks]
21-40 Adept Mastery [C rank attacks]
41-60 Skilled Mastery [B rank attacks]
61-80 Proficient Mastery [A rank]
81-99 Excellent Mastery [A to S rank]
100 Supreme Mastery [S rank]
The attack power will depend on mastery. Let's take Luffy fruit for example
His attack name,
Gomu Gomu no Pistol: In Order to execute that attack you need 21 Devil fruit Mastery, 21 Strength
Gomu Gomu no Bazooka: A attack that usually used for finishing move for a basic fight can be considered a strong finishing blow, and in order to execute that you need 44Devil Fruit Mastery, 41 strength
And so on, every attack according to Its Destructive Potency you can decide it's rank and how many points you need in order to execute that attack
I can explain this in more details but don't have time right now.xD
The Ideal level of RP is 10 but keep in mind when you make a new RP, you need players who should be strong from the beginning. You can't start from the scratch when there aren't any high-level players already present to lead the RP. But yeah, before giving new player level. You guys should start a competition
- Like every participant of that competition gonna start from level 30
- And Winners of that competition gonna start from level 45 or 50 in order to balance things in the beginning.
- So whoever came after that competition will start from level 10, you can call them newbie
- Having numbers doesn't mean we have to use those numbers. Even a weak NPC level start from 10 and before that number you'll be considered a dead weight for fights
You can make custom devil fruits but keep in mind that it needs to be balanced, so it can be used properly. Like a teleportation ability will be too broken in One Piece RP like Trafalgar Law ability
About the first question: It should be different than Naruto RP so players can experience something new which give them more reason to play in both RP for a different culture
One more thing, before making a One Piece RP keep in mind that it will require a lot of time, patience and too much-written stuff, rules, proper use of stats, etc