I'm using this stand this weekend, but I need to put the longest legs on the 12" circle. Has anyone done this? Any structural problems with doing this? I'm putting a 2 tier cake(10" and 8") on the plate.

How are the legs attached? Are they screwed onto the frame??

Yes they are screwed into the stand and tightened with a small wrench.

As long as there isn't a weight limit, I would think you'd be ok using the long legs. I would probably remove the top tier before serving, pushing down to cut the cake might put a lot of strain on those legs.

Put the long legs on and then put some books with the equivalent weight on it and test how sturdy it is.