Those do not look all that healthy, IMO. It appears almost like they are grown inside with nowhere near enough light upon them...they are indoors, full-time, right?? A silk-tree that cannot hold itself up is *not* a happy tree at all. The lower trunks really look rather puny, no offense intended at all, and really needs to be addressed rather quickly before they run out of stored energy and simply flop over exhausted and depleted of any future hope.
There should be no need whatsoever to tie them to a 'stake', even if in an area of high-winds. Mine always withstood tornadic winds without any effort. If the need for staking (or 'strawing', LOL j / k) is there, the plants are
*super* weak. Silk-tree is super-vigorous in a proper environ, and never is 'floppy' at all (as all those look, IMO). Typically, the internodes are *much* longer, and for some reason, those jsut are not growing like they should (and not from overlighting, I promise, LOL).
With you saying you have given some away to friends, I'd make sure that they are not 'outlawed' for in-ground use in your locality/State (www.invasive.org is good place to start) to make sure that no one gets in 'trouble' naively. Some municipalitites come down pretty hard on these plants, fwiw. I'd double-check on it myself
I also have mesquite seed that I am thinking of starting this Spring, and have several other harder-to-find species with similar foliage. I have lots of Black Locust cuttings of various caliper successfully rooting right now, and expect to have some nice trunks going next Spring (crossing fingers!). I have always thought that a 'forest planting' of ancient-looking 'trees' (dinosaur era, per se) would be neat - even use Norfolk Island pine for the pre-historic look seen in artist-drawings of dinosaur-like forests you always see depicted in museums or wherever.
Also, please describe the soil used - it makes a huge difference, believe me...
(edit) if you wanna PM me your e-mail addy, I'll gladly shoot ya a few small jpg's of my silk-tree that is less than a year-old to show you what they *should* be looking like. I have a few pics when mine is about same age as yours, and some 'current' ones as well. Huge diff's in appearance between mine and what you show
Not implying I am better, just the environ yours are in is rather unworthy for this species.