It's open to characters from any publisher - not just DC. However, if you want your story to be considered canon, send a PM to Lilith beforehand - she's coordinating the universe and keeps a light-touch regulation on it. She maintains the list of characters who've been 'claimed' as well as a handful of vetoed characters (e.g. Superman). It's also a good idea to steer clear of overpowered characters - besides which, if you keep strict limits on what your character can / cannot do, it will make a far more interesting story than the hero/ine who turns up to a disaster, then five minutes walks away with a smug grin after clearing it up single-handedly, before flying back to be ready to teach 5th period history. Although the vast majority of characters are M2F transformees, that is an optional aspect. For example, Alison "Copycat" Scott is a girl from birth, and the CEO of Wayne Industries (plus part-time crimefighter) is male from birth. Your characters doesn't even have to have any superpowers - we've got a few 'baselines' in the mix as well.

Most CRU stories are organised within the same 'book' on site (look here), so you should be able to get a rough idea of what's already been done. Needless to say, if you're planning on using any characters other than Jade, it's a good idea to have a chat with the relevant author. Also, look at the Comics Retcon Wiki - that contains the timeline of when things happen, so you don't fall into the trap of using characters before they've appeared or when they're otherwise engaged; as well as an ever-growing character list (not just the heroes/heroines themselves - but also regular supporting characters).

A final note - Witchblade has already been claimed - Colder Than a Witch's... Part 1 and Part 2.