You don't actually have to care much about hard-core physics.
If your sci-fi world is set in another universe entirely (Star Wars and the Force), you needn't even bother about following the periodic table or conventional physics. After all, the Force isn't even something the Jedi (in their Universe) can explain completely.
It's okay to create imaginary material with the materials we currently have in the periodic table. Avatar (the movie) is set in the future in this universe, and the filmmakers decided to introduce Unobtainium, a material based on sci-fi physics (see http://james-camerons-avatar.wikia.com/wiki/Unobtanium).
If your universe is not anywhere near Earth, than that just makes things easier for you. We think we've got the laws of the universe figured out, but who knows, really, what will happen under extreme conditions not found anywhere near Earth, especially when we do not have the equipment to reproduce them? Example: Albert Einstein said that black holes were ridiculous when they were first proposed. It sounded ridiculous; a mass so great that not even light could escape it, a mass so great that it would crush itself. My point is that you do not even need to look too far from our current laws of physics: Just bend them to the extreme, and say whatever you want. An actual example closer to home would be the fact that the Sun's corona is millions of Kelvin hotter than its surface (Coronal heating problem).
Scientists still haven't come up with a theory to explain the effect (as far as I know).
Screw it all, there are even things in our oceans that have our scientists' jaws on the floor (http://www.dailymail.co.uk/sciencetech/article-2081787/Hydrothermal-vents-Lost-world-unknown-species-Antarctic-sea-bed.html) :
They were exploring off the coast of Antarctica and found colonies of marine life including crabs, an octopus and starfish totally new to science, living in the murky depths.
The reason their existence is remarkable is that they were found on top of undersea volcanoes called hydrothermal vents, which pump out plumes of black smoke causing temperatures to rise to 380C - hot enough to melt lead.
In a word: Don't worry too much about having to follow mainstream physics. Science Fiction is about what could happen, not what will happen. If you're writing Science Fantasy (even more whacky than Science Fiction), then you have even less to worry about.