Theoretical possibilities are a writer's best friend.
Let me expound on that: something does not have to actually exist for you to use it in your story. It merely has to be theoretically possible.
For example, as I said yesterday, there's no reason for anyone to build a working steam cannon. Why? Because gunpowder-based cannons do a perfectly serviceable job already. However, the fact a steam cannon is theoretically possible is cause for celebration because that means it can be used with a minimum of fuss.
Similarly, knowing that Leonardo da Vinci theorized about many different inventions during his lifetime means that we can postulate what Renaissance Europe would've looked like if someone had the presence of mind to build his inventions instead of them merely languishing on paper for 500 years. Interesting concept, yes?
Steam technology was first theorized by Hero of Alexandria in the first century A.D. His invention, the aeolipile, was the first steam engine. Consider this then ... what if the Industrial Revolution happened in 1st Century Greece instead of 18th/19th Century Great Britain and the United States? Great fodder for a steampunk tale, isn't it?
This all of course leads back to the fact no one's expecting your fictional technology to actually work. Just that it has the potential to work. So long as the tech abides by rules you lay down, then you're fine.