I have currently have a Canon 550EX and am wondering if it can be triggered with this transmitter. I'm still trying to understand the technical pieces of using flashes and wonder if someone could let me know.
The -RT triggering system is much newer than the 550EX, and I think the only way you could trip one is if you added a YN-E3-RX radio receiver to the foot of the 550EX. But some B&H and Amazon reviews say that this combination does not work, and the 550EX is not on the list of flashes that are compatible with the trigger.
You cannot use the built-in "smart" optical slave in the 550EX, because the YN-600-TX is only a radio master, it is not an optical master. And the 550EX has no "dumb" optical slave to be set off by any flash burst.
You could use a YN-600EX-RT II as a "smart" optical master to the 550EX, but again, how much control you'll have will be more limited than you might expect, as the 550EX is only capable of groups A and B, and can only be controlled by TTL ratios over the "smart" optical system, not manual power ratios (1, 1/4, 1/16, etc.)
If you still want to use the smart optical system, it might be cheaper to get an ST-E2 (which is the same era gear as the 550EX) or the Yongnuo clone of it. On the used market, these should be very cheap, since most folks prefer radio triggering for remote flashes.
You could also treat the 550EX like a manual-only flash put it in M, dial in the power setting, and use manual triggers, like the Yongnuo RF-603II or Phottix Ares.
I'm sorry to say it appears it doesn't support the Canon 550EX.
I got this info for you from Yongnuo's website: The YN600EX-RT II now supports both Canon and Nikon optical transmission wireless slave flash.
One YN600EX-RT II can respectively receive the wireless signal of master unit YN600EX-RT II, YN568EX II, 600EX(II)-RT, 580EX II, SB-910/900/800/700, Nikon built-in flash C command, Canon 7D/60D/600D cameras built-in flash wireless signal, realize wireless TTL, manual flash and stroboscopic flash.
You can't trigger the 550EX with the YN-E3-RT.
The built in wireless receiver capability of the 550EX is Canon's optical control system where a series of pulsed flashes (at very high speeds - it looks like a single flash to your eyes) communicates with the flash to tell it when to fire and how much power to use.
The YN-E3-RT uses Canon's much newer radio communication system to communicate with flashes (or triggers) with radio receivers compatible with the RT system. There doesn't seem to be an add-on wireless radio receiver compatible with the Canon RT system that will work with the much older 550EX to give you E-TTL and remote power setting capability.
If your camera has a PC port (it's a flash connector that has nothing to do with personal computers), the simplest (and maybe only) way to use the two new components you are considering together with the older 550EX is to control the YN-600EX-RT II with the YN-E3-RT mounted on the camera's hot shoe and fire the manually controlled 550EX via a cheap set of manual radio triggers with PC triggering¹. The transmitter would be connected to your camera's PC port. The receiver would be attached to the 550EX's foot. You would need to set the flash power of the 550EX manually using the flash's own controls.
Of course you can also use a Canon optical controller, such as mentioned in this answer, to control both the 550EX and the YN600Ex-RT II. But you would be giving up the considerable advantages of radio over optical communication.
The difference between an optical communication system, such as that used by all Canon master flashes (those with an "EX" but not an "RT" in the model number) prior to the 600EX-RT (and the subsequent models with "RT" in the model number), and a radio communication system such as that used by most wireless triggers:
- Radio tends to have a greater range than optical.
- Radio doesn't require line-of-sight like optical does.
- Radio can operate in very bright environments such as direct sunlight that gives the optical system a tough time.
- Radio has the ability for more than one set of the same type to be used in proximity to one another without interfering with each other. (Think several press photographers all using Canon covering an event for multiple publishers. Or more than one shooter at a wedding.) If you have more than one photographer in the same area using the Canon optical system, all of the flashes will react to all of the controllers.
¹I've used the 'Cowboy Studio' version of this same trigger set. The transmitter uses a 23A 12V battery that is currently popular for key fobs, garage door openers, baby monitors, etc. Ignore all of the reviews that say you can't replace the transmitter battery. It is a one screw removal/replace job.