Builds do NOT need to be set in stone. However, try and place yourself at maybe an SL70 sort of character, weapons and powers wise. At level 70, you will not have a kickass chikage with a logarius wheel and throwing out Call of Beyond while your at it. Your character does't need to have stats, but they cannot use everything the game has to offer. Multiple characters are a good thing :)

Custom weapons ARE allowed, as long as they fit the time period. For example, if you want your character to have a standard rifle, such as the one that the Yharnam citizens use, go nuts! Just remember that it should fit the atmosphere of Bloodborne. Sunlight spears and such are probably NOT going to work.

You do NOT have to wake up in Iosefka's Clinic like you do in the game, nor do you have to be one of the immortal hunters in the Dream. However, I'm going to say you cannot roleplay as NPC's. OCs are required, but otherwise you have free reign in that department!

I think when the community starts to get big (however many people that is), invasions will be a cool thing! Especially in one of the invasion based covenants, which is pretty much all of them sorta not really. I think having hunters pop up and try to murder you is great! However, that brings the laws of RP fights into play. Something along the lines of "I popped up and shot you with my bone marrow evelyn and you died" will NOT be acceptable. I will void your post and ask you to edit it if that does happen.

Also, on a similar topic, summoning for co-op probably won't be a thing until the community grows larger too. When it is just starting, as it is now, everyone will be in the same world. Whether you fight or work together in that world is up to you.