Hi guys... I've been away from the Forum for awhile so I didn't catch this thread until now.
First off, I really appreciate the kind words, and it makes this project all the more worth it.
As for compatibility, the only compatible rails that I know of for sure are listed in my FAQ. I hate to speculate about rails not listed such as BSR's Ulimate rail and the Unistrut extrusions (although I'm pretty sure they're not compatible). I did recently receive word that the "Xtreme" rail might be compatible with my larger buttons... I'm waiting on photos to be sure. (Not to be confused with Giant Leap's "Extreme" rail, which is compatible with the smaller buttons.)
As for which button to use for your L3... the deciding factor shouldn't be the button, but the size of the rail required to support the rocket. If the BSR HPR rail or 80/20's 1010 rail is strong enough to support your rocket, then the smaller delrin buttons will work just fine (my nylon buttons have even been used for L3, but I don't recommend it). Otherwise, you'll have to choose your larger rail based on what's available or what you're willing to purchase yourself. I'm pushing the 80/20 1515 rail, but there's also the BSR Ultimate rail, Unistrut extrusions, Xtreme rail, and various other home-grown club-specific rails.