Omega Supreme has ports on his track which are compatible with the bases in Titans return including of course the other Titans Trypticon and Fortress Maximus. This is so that fans and kids can build their own massive citis with the huge bases. You can see those connecting ports on the image below.
Jetfire also has backwards compatibility but with Prime Masters and Titan Masters. Both of those smaller kinds of bots can sit in Jetfire's cockpit.
These are just fun Easter Egg type features that will not be documented since they are meant as a bonus to fans who already have figures from prevous lines. If you don't have those previously released figures, don't worry there is a lot to enjoy from these fgures which each have their own special features.
Another point regarding Omega Supreme, John Warden did confirm that his bot mode is meant to scale with Devastator.
Let us know if any of this info makes you happy and if you feel Hasbro is taking things up a notch with their Transformers toys.
I know I've been critical, and I'll continue to call out things I feel are glaring omissions and missteps, but I'll try to do better to give props when do. This is about the best outcome one could imagine. I fully intended to make Siege Omega Supreme fit within my giant city network, prior to this announcement, but this just makes it all the more satisfactory. Also I fully intended on getting Jetfire, since I never got the T30 one, and this one is actually (seemingly) better...The TM/PM BWC wasn't necessary, but is certainly welcome. These little things definitely do push the RIGHT kind of buttons with and old fuddy-duddy like myself. 
