Freesco is listed on that section of the page, along with some others.
The only tiny distros I've used have been Basic, Mu, and Floppix - well, I more "booted" Floppix than "used it:, which is true of PicoBSD, as well. Can't say how they'd do for your purposes, but Basic is a pretty nifty all-around floppy distro that can be transferred to hard drive.
There's this, which is broader than just firewalls. If you wanted something Red Hattish but not Red Hat, it sounds like ClarkConnect might do you.