It's completely acceptable!
We regularly create...
- Fictional companies within existing Earth cities.
- Fictional cities within existing Earth countries.
- Fictional countries on Earth, which I propose exists.citation needed
- Fictional stars, planets, and locations within the Milky Way.
In fact, it's often much simpler to start with something that exists and modify it to meet your needs. New authors are often caught up in the details, not yet having learned when focus is needed and when it's not. Inventing an entire galaxy, for example, just to tell a story on one world (where all you may need is to define some constellations for that world), is a ton of work without value — unless you intend to write a slew of stories within that galaxy.
But even then, it's frequently better to invent it on the go. Terry Pratchett's Discworld was invented that way — and sometimes he changed the rules. The Discworld of his early stories is not in every detail the same as in his last.
So, modify away. Build only as much as you need and move forward with your story.
One last point. While I wouldn't be at all surprised for an enterprising lawyer to step up and claim to represent the Universe's best interests. I suspect you might not have deep enough pockets for one to be well enough motivated — and I'm fairly sure the Universe, itself, won't mind too much if you copy its structure for the sake of your story. :-)