And I just wanted to be sure that all of my older USB-2 devices (e.g. external HDD case, hot spot, phone headset) will work on my new MBP
The current MBP doesn't have USB-C so that won't affect you. When the rumoured new Macbook Pros appear, that may change, but for the moment USB 1/USB 2 devices will plug straight in to the USB 3.0 ports on the 2015 rMBPs.
The familiar large, flat USB 1 & 2 plugs and sockets are actually called USB-A connectors. The squarer ones that go into the back of printers etc are USB B connectors. The USB
3.0 A-connectors (& cables) are actually different from the old USB 1/2 ones in that they have an extra set of pins buried deep inside the socket, behind the regular ones. However, old-style USB connectors still fit & work.
You now also have USB 3.1 gen 1 (the same as USB 3.0, 5Gbps) and USB 3.1 gen 2 (10 Gbps). The current rMBP doesn't support "gen 2". These can use the same type A connectors as USB 3.0.
USB-C is a new
connector which can be used to carry some or all of USB 1, USB 2, USB 3.1gen1, USB3.1gen2 as well as DisplayPort, HDMI and Thunderbolt 3. It can also carry power (more current than USB-A could). However, not all USB-C ports on devices will do all of those functions- e.g. the 12" MacBook has a USB-C port but doesn't do USB3.1gen2 or Thunderbolt (or HDMI?). The proverbial new Macbook Pros are expected to have USB-C/TB3 ports that do the works.
You'll be able to get adapters for all of the old-style connectors, but long-term things like displays, disc-drives etc. will start sprouting USB-C ports. They've announced a quite bewildering array of cables which hopefully will get thinned out a bit in reality. The important ones will be USB-C to USB-C cables that do all the USB protocols + power + DisplayPort, short passive Thunderbolt 3 cables (probably just thunderbolt-certified products that are physically the same as the regular USB-C cables) and longer, (& more expensive) active Thunderbolt 3 cables, that just do thunderbolt.
