Round or square tubing? Either would work for a mini.
I used to design GP racing frames for 600-750cc class for a few AA racers, mostly girls since I made the bikes a lot smaller for them to ride. The fixture would be VERY simple since they are a mirror.
You make the fixture so the side of the bike would be flat against the fixture. You weld a complete side. Once the side is complete, you remove and welded another one, this is the opposite side.
At this point I made an upright. This was just a system of holding the cross supports horizontally. You clamp the sides to the horizontal supports and weld them in place and you are done.
To be honest you wouldnt need a roll table, unless it is an over sized bike. The frame would be so light you could just flip it up on end and weld the other side.
Matthew