As far as using stock media goes...there is nothing wrong with using it (even if the sight of it makes us old timers puke...most people don't know about FPSC and the stock media is decent).
It's what you do with the media that will make it a great game.
"Shoot everything that moves" has gotten old.Gamers are growing increasingly more sophisticated and more demandikng in what the want out of a shooter although killing is a part of it.
You might want to eek out a story outline...just to give you an idea of what you want to do with your game.
Make your rooms a bit smaller,add more media and some lightmapping.
And 25 enemies in one room is not a good idea...while x10 may promise instancing and that many enemies in one room this version of FPSC most certainly cannot make that claim.