The good news is that you have lots of options, but that's probably the bad news too 🙂
The easiest answer is that yes, you can get an adapter that slides into the hotshoe and has a PC port if you want to go old school and use cables. It's simple enough and cheap enough, but there are much better options.
One option is to use a flash as a master flash to signal the others. The 550 can be used as a master, but your 70D's built in flash should be able to do the same. But you'll only be able to use the Canon speedlights using this function (unless your monos have an optical sensor, but you can buy these). You can also set it so that the flash fires to trigger the other flashes, but doesn't affect the exposure.
The option above is dependent on line of sight, which can become a nuissance if using modifiers. Using radio frequency triggers gets past this limitation, and is the standard method these days. If you're ok using manual only (the monos are only manual anyways) you can get really cheap triggers. Pocket Wizards are the gold standard, but expensive. Yongnuo is the 'gold standard' of generic options. I highly recommend the RF-603 II. They're about $30 for a pair, you'll need 5 total (one goes in the hot shoe as a trigger, the other 4 will trigger your 4 flashes).
There's a lot of good information out there, as you've found on YouTube. I highly, highly recommend going through the Strobist blog. It's geared towards using speedlights, but there's some discussion of studio lights, and much of the information is universal anyway:
http://strobist.blogspot.com/2006/03/lighting-101.html
