Well fixies tend to put a lot more stress on the rear wheel, dropouts and driveline than a single speed freewheel. Requiring a completely different drive train to be safe and ride good. To make a fixie you need a bike with long 2 inch or so heavy horizontal dropouts. Preferably with a nice strait gage lugged steel frame as opposed to stiff double butted cro-mo frame. Plus you need to change or rework nearly every part on the bike for it to work good. If you just need a bike to ride go with the bike you have as a single speed. If it turns out you need a chain tensioner you can just use an older derailer on the rear free hanging and adjust it to your chainline.