Yeah, you can make any pokemon you want as long as you have the correct version of Pokesav (there is one for Diamond and Pearl then a different newer one for Platinum)cbecause of the different pokemon you can catch (it's just pretty much the new form of Giratina and the Rotom forms) and a few new items.
I'm not too sure on this but I think you can alter the pokemon you already have instead of comepletely recreating new ones but I've never done that before so as I said, i'm not too sure.
It depends which community you play in, I sometimes battle people from Smogon under the name Durion but I have to tell each opponent I am using Pokesav'd p be okemon (which I have just written in my signature so I don't have to tell everyone anymore =D) so they don't mind. The only rule that most communities use (well I'm guessing because this is true for Smogon, there may not be many other sites that allow Sav'd pokemon though) is you have to keep the pokemons stats legitimate, it's infomation legitimate (e.g. where it was caught, ID No. and Trainers name, Pokeball...etc) or it's movesets are legit (so make sure you don't make a moveset with a Egg Move from the 4th Gen but a Tutor move from the 3rd or something like that).
Sorry the rather lengthy paragraph on Pokesav, also if it doesn't make sense in any part just ask me about it