If you make sure that cog is on there using automotive engine grade locktite, you'll be okay. Once you thread the cog on and use the appropriate adhesive let it dry, and then use a bottom bracket lock ring as another form of "insurance" it'll be bomb proof.

Using a free wheel type of hub is okay for fixed gear, you just need to know how to make it so things won't come apart on you. I know plenty of old timers who did just that for years.