l4justice, what you describe as "using a base" can also be called intellectual theft, plagiarism, or, most simply, art theft. Such is absolutely frowned on in the art show and also in the artist alley, as both are devoted to showing
original works. If another artist, whether syndicated or not, takes the time and effort to find a unique way of portraying something, that is his or her individual intellectual property. Anyone who traces over what someone else has created and slaps on a different hair color or something, and then tries to pass it off as his or her own work, is committing theft. In other words, it doesn't matter if you are copying an "official" artist or some random kid on deviantart--theft is theft. This can also be applied to 'eyeballing,' or copying without tracing, someone else's work. Any art in either the AA or the Art Show that can be justly proved as a copy or a ripoff will be kicked out. I don't mean to be overly harsh, but as you have probably heard,
art theft is serious business. That said, there can be merit to copying other artists' work to improve. Even most art schools have the students attempt to copy the work of the masters. The point is not to steal someone else's base and alter it slightly in the hopes of making it
yours, but to learn to improve your own abilities. Copying another artist's innovation can be a great way to begin to understand the human body, light and shadow, or anything else you may be studying. There is nothing wrong with doing this as long as it is kept to private study, for your own benefit. Just don't pass it off in public as completely your own work, and
certainly don't try to profit from it!
Now, there is a difference between being inspired by someone and simply ripping them off. The difference, as neatly as I can put it, is whether we can tell or not. If you want to imitate the way that someone paints clouds, or draws hair or something, that's fine, as it is you
doing your own work. It may remind us of so-and-so's style, in the same way that Natsuki Takaya could remind us of Watase Yuu, but it is clearly not a blatant rip-off.
If you need any help being pointed in a good direction to improve your study of art, or if you have any other legal or ethical questions about it, I'm happy to help. But for now, I hope that answers your question.