I currently have a free wheel single speed road bike and have lately been interested in trying a fixie, I am sold on getting a flip flop hub but I was curious if I can just buy a fixed cog and just add it on the singlespeed hub? (Also next to my single speed cog about an inch or two on the same side is another cog and looks like a fixed cog but gives too much slack when the chain is on it) thanks!!

Highly ill-advised, that is what is called a "suicide hub" for good reason.
Only a proper track style hub with a reverse thread lock-ring to keep the cog from un-threading under pedaling back pressure (with inevitably very bad results) is safe to use on a fixed gear bike.

"Another cog on the same side?"
"Too much slack"?" Post a pic.

Inexpensive rear wheels w/ the proper hub are readily available, get one w/ a flip-flop design to use a FW to be most versatile and safe.

-Bandera