Low Top Tube Xtracycle Donor Frame? One of my most useful bicycles is my 58cm 700c Xtracycle based on a Novara Xpresso cyclocross style frame. Lately, as the kids are getting bigger and my back isn't getting any younger: I crave a much lower top bar. I would consider a used mixte frame, a step-through frame, or other really low diamond frame. Recommendations for sturdy and faithful frame models appreciated. Ability to handle 3 rings and 8 cogs plus a chain stay bridge preferred.

These areas would help me evaluate the frames I might find: 


Stability: I have heard some people mutter that mixte frames are "unstable" as donor frames. Is this substantial? (I get some shimmy and flex in my present Xtracycle but I'm running a cro-moly frame but it doesn't bother me.)
Fit: Is there a more limited range of geometry among these frames? Could I find frames like these around 56-58cm or about 19.5 inches? If it is a large "WSD" style frame, how much difference does that make for a male pilot?
Strength: Are frames with lower top bars any weaker?  Steel is not a requirement, but I expect to load it up with camping gear and elementary school
age kids.
Have a look on the second hand market for 'Giant Trail' mountain bikes from a few years ago.  These had an unusual frame design with a lower top tube, wider, more stable bars and disc brakes. Giant Rincon Trail, Giant Boulder Trail were a couple of the affordable models I remember.