Sci-fi story help!

Okay, so I'm planning on writing a sci-fi story, but there are some things I need to work out, and I want to know if anybody has any ideas.

Here's what I have so far...

The story is about an alien race that lives on a planet which is sort of like ours, but inside out. The core is at the center of the planet as usual, but gravity is reversed, and the alien race lives on the inside of the planet, so that when you look up, you see the core off in the distance. The core acts as their source of heat, except that they live much closer to the core than we do to the sun, so they are heavily resistant to the intense heat that radiates from it. However, because the core is always shifting, sometimes gravity will be stronger at certain places that others. The alien species must also be able to resist this quite well.

I need to create a physiology that would meet those two conditions, but everything I think of is based off something else. Help?

NOTE: I have an idea of the plot, but I have no characters. I need to build the environment before I can put beings on it.

Well, before getting into physiology, I don't really get what you're saying here... the planet is inside-out... and yet the core is somehow floating off in the center of the planet and then there's... air? And another layer of rock so that it's sort of like you have a beach ball or something, with people colonizing the inside of the beach ball's "skin" and a sun-like "core" floating in the very center of it all?

That's some pretty impossible geology going on if so, unless you're going to make the core totally different from the kind of core at the center of, say, the earth, because if it's not actually the center of the planet's gravity (i.e. the outer "skin" of rock would need to be incredibly dense or thick or something in order to have enough mass to exert gravitational pull on the core from all sides) and having all that mass crushed down upon it, it won't be in conditions anything like at the center of our planet. If you're trying to have this

In any case, the core shifting around wouldn't cause such dramatic swings in gravity anyway--consider that tides are caused by the moon and sun, and that's as dramatic a change as we experience based on those huge celestial bodies swinging around up there. Heat resistance is fairly easy, depending on how much heat resistance you mean and what is available in the environment for animals to utilize; without getting a clearer picture of the world you're talking about, I couldn't say much other than that something so simple as being subterranean would help you out there. There's loads of ways to deal with heat.

The concept of the world itself sounds a bit shaky to me, though. For anything supposed to be halfway realistic, I mean--you could always throw out "a wizard did it," of course, if you were going fantasy. It's not unworkable, but it is going to take a lot of thought.

Sounds a bit like the Ringworld series only, and I mean this in the nicest way possible, worse. =/ Negrek basically nailed all the basics; the science of this idea just doesn't really work. I mean, at a push, you could explain it all with a kind of "it works because I say it does shut up" kind of attitude, but you'd have to have an exceptional storyline to back that up for people to forgive your shaky science, especially if it's what this entire world and its happenings are based around.