Default MTU for ethernet is 1500 bytes.
The reason DSL needs the MTU changed to 1492 is so it can rob 8 bits for an ATM header. Some backbone network you are crossing is requiring a smaller MTU, start at 1492 and work your way down until it works, the biggest one that works will provide the best performance.
Ideally you want the router to take care of this (or educate everyone on how to change their MTU, Dr. TCP is the easiest way to do it.)
I do not know if any of those devices can modify your MTU, I am guessing they cannot.
Beware I am a Cisco zealot, but I would go with a Pix 501 to get a good firewall in place as well as modifying the MTU. Oh yeah, fire your curent vendor.
My two cents, whatever it is worth