I've come to the conclusion that aluminum is not a good build material for a folding bike with small wheels. The Swift/Xootr is the only aluminum bike I've seen that doesn't have gobs of reinforcement in the BB area, where the forces generated by the long top tube act as a flex lever and along with the forces generated by pedaling, necessitate great strength. The Birdy, in particular has a massively overbuilt top tube and four weld/contact points at the bottom bracket; since it doesn't fold, it feels as though it may last (at least in the main frame. The front and rear suspension assembles both are suspect for me now).

Someone mentioned standardizing the seatpost tube size and I'm sure you'll find that you must shim a very large tube here, again because the forces generated are far higher than on an ordinarily sized frame. Steel and titanium are the options most likely to work and, given the strength requirements, steel might be a weight competitive choice. Check oyut the Bike Friday line for a good selection of lighter weight steel frames.

On a note of personal choice, I like to fold my bike in half, on the horizontal plane (like the Dahons and rather dislike the gymnastic effort required to fold bikes that have the rear wheel swing under. I refuse to actually fold my Birdy simply because it's such a messy folding design.

A refinement of the handlepost would be welcome and don't forget to study a little history. There've been som very interesting designs put out in the past, like the Panasonic/Soma ti journey.

Good luck,
DG1