It's up to you what universe you want to bring in (even if it's an OC universe). There is only one universe we don't allow, that being the Bleedman universe. If you don't know what that is for the sake of your own sanity do not google it.
Eh, can't be much worse than Boku no Pico. I'll probably use my Undertale OC just because I can. Plus side is he can timeline and universe hop anyways.
I meant the fact that he technically can. That doesn't necessarily ever mean it will ever be used. It's tacked onto those powers he has but is never used because they have no use.
Depends on which version you want. There's MAD (he's the much stronger version), and DT1. All abilities are negotiable and only are there are just defaults.
Only real difference is that MAD has no moral compass. Technically speaking in their own personal canon MAD is much more powerful, but they're both made to be adjusted to different settings accordingly.
I dont know what you're expecting. You're literally not giving any ideas or even what universe the RP will be in. You have to give something to get something in return.