It's pretty tricky answering this question for me...

I'd say keeping the same theme in different stories has it's strong and weak points, strong points being, you're really good at those kinds of stories, and you know people are going to enjoy it, on the downside...it's the only theme expected of you, so it can scare some authors into blocking themselves into that theme, and not venturing out, which I think is a stifling of creation.

As for recurring species...I can only say that it allows people to assume that they're your favorite, and unless those species are their favorites too, they'll get bored of the stories quickly...I think.

As long as you don't do it too much.

And please, for the love of GOD, do NOT become Nathaniel Hawthorne and write the same story over and over and over and over and over and over and over again, or take three chapters to say something that could be said in one paragraph.

Recurring species is okay, so long as it's not the main character every time, and recurring themes are only good if you know how to write them well. I dare say you could do that, given the sheer amount of talent and creativity in that head of yours.

If you do it well and work in variations this can be a very interesting thing. If I might borrow some examples from film I one could look at Peter Wier's films, which look at people who are out of place, at Terry Gilliam who looks at the definitions of sanity and reality, or Hitchcock and his study of ordinary people caught up by accident in large events. All these directors have produced wide and varied cannons, the films are very different because the thread each director likes to explore has many different ends and stories that can be told. So if you do follow a theme it has to be a wide casting open-ended one, but this can be fascinating!

In terms of continuity, such as a squeal or a prequel, it would only make sense to use them granted they are story specific. However, Let's say you release a hit single that has a catchy tune and a familiar subject matter. But then, you're next album comes out and the single you release for it follows the same basic format as the previous.

In short, be aware of what you're doing, reuse only if the storyline calls for it, too much repetition creates anticipation and kills interest through predictability.

That's a tough one. I do have a lot of recurring themes in my stories, and a great many of them are, in fact, points of weakness for me. It feels like a lack of creativity on my part. My empathy button is kind of broken, so I have a great deal of difficulty writing from someone else's perspective. With the old advice being "Write what you know..." the main character ends up always being me. Sure the minutiae change of the character's backstory, who he actually is in the narrative, but his personality always very closely mirrors my own. Sort of a sharp, mellow, good-natured cynic with an answer to everything, correct or not. This does limit me, as I can only really see the world through one pair of eyes. I'd love to get the experience of someone more hopeful, heroic or passionate or what have you, but I just can't fabricate those qualities convincingly in demeanor, thoughts or mannerisms, the things that I use to characterize the main character.
The result has been a rather narrow view of my various fantasy worlds. If you don't like the deadpan anti-hero type, you're pretty much out of luck in my writing. But it really wasn't all bad. This self-insertion role-playing style of writing led me to try writing in first person, wherein that is essentially the objective. My familiarity with the character (me) makes the story much more immersive no matter how much you think he (I) acts like a pillock. And above all, I felt comfortable while I was writing, I could explore and manipulate these traits easily because I knew them so well. It the end, this failing of mine resulted in some very good stories, and very flattering versions of myself. ^_^ I guess what I'm saying is, if you're good at writing a certain thing or using a certain element, do it. Everyone loves watching someone do something that they're good at. Base your work in what you know, then experiment with the smaller details. Tinker with the minor characters, have a subplot about some espionage or other intrigue that you wouldn't normally use. And if you find something else you like, another thing that you're really good at, add it to your toolbox. You're going to need it later.

Depends on the story, I say.