It will run better with more, since there's less swapping going on with more memory, and it'll go faster because of this. Whether this is playable or not will be up to you. It will run, but whether the reduced amount of features is acceptable is a question we can't answer. Only you can answer that. You cannot join games in progress.
All players have to be there when the game starts.
If you quit, you can't get back in. Shareware has have 5 levels and 1 secret level. Number of levels in registered is as of yet undetermined. There will be 6 weapons in shareware, and 9 in registered. We respectfully request that you not make any addon programs that work with the shareware episode.