Is mixing elements of Wicca with Fictional Witch Craft a good idea?

There's nothing wrong with including characters that practice a real-world religion in your fiction.  But it is important to depict it accurately (as you are clearly attempting to do through your research) so as not to turn off a large segment of your readership. 

Something to bear in mind is that Wicca is a rather diverse religion, meaning that considerable variation exists in both beliefs and practices.  So it may be a good idea to make it clear to the reader that your characters with their "heavily tweaked characteristics of fictional witches" are following their own specific interpretations of Wicca instead of being representative of the religion as a whole. That way Wiccans who may not agree with how your characters follow their religion hopefully won't feel that you're misrepresenting it.

You are writing a work of fiction. You can have your characters do what you want.

Ephesians 5:11-12 

And have no fellowship with the unfruitful works of darkness, but rather reprove them. [12] For it is a shame even to speak of those things which are done of them in secret.

Personally I don’t understand why you would try and glamorize something that is real & is very much evil. Dabbling in evil, even just for a story is bad luck my friend. Stick with good vibes. Evil energy is real. 

It doesn't matter. It's all fiction dear.