If you want to connect MIDI Out to multiple devices’ MIDI In - you will most likely need an Y-type MIDI cable like the one here - http://www.gak.co.uk/en/tc-electronic-m … able/49990
If You want to do the reverse - connect multiple devices’ MIDI In to the single device MIDI Out, this generally won’t work. Probably the best answer for this situation is MIDI Solutions Merger - https://www.gearslutz.com/board/midi-so … ger-g7989/
The MIDI chain you posted looks ok. The one thing that you should always be aware - the more total MIDI cable length is the more likely some delays in MIDI synchronization will appear. To put any real numbers here, I use 3 meter MIDI cable to connect BeatBuddy to my Boomerang III, and there are no problems.
I would connect MIDI like that BeatBuddy > Boomerang III > Rang Side Car > all your other devices one by one. (“more” sign here means connect MIDI Out of previous device to MIDI In of the next one).
According to specs, Boomerang III will definitely pass tempo through (unless its powered off, of course).
So what exactly is your problem once again? Not all of your devices support MIDI daisy chaining, like not having both MIDI In and Out ports? If that’s the case, I think Y-type MIDI split cable is your solution.
In this MIDI chain, we are using BeatBuddy as MIDI Master device. Master device is the one, that generally controls the tempo. It is not a good idea to use two MIDI Master devices at once. So if you are used to using DMC-8D as the one that controls the tempo, you can try connect it as the first device in the MIDI chain (connecting DMC-8D MIDI Out to BeatBuddy MIDI In), effectively making BeatBuddy a Slave device.