ive ran servu since winter of 98 ...
never had to set it up behind anything else though ...
i'd think all you would need to do is change your firewall to allow for the port you chose your FTP to be on is open.
and if you are using yer linux box as some kind of router or proxy .. make sure all the ip information is correct too ...
it probably should not be too tough ....
but im no expert with linux .. i only ran it for a few weeks ... and i missed my windoze so i went back .. (linux was neat ... but not what i needed)