Just to clarify, it isn't a mini dvi port, its a mini display port (mini DP).
1) The thunderbolt port is backwards compatible with display port, so using your dp adapter, and the usb monitor will be fine.
2) what everyone else is referring too (which may be confusing) is that thunderbolt in the 15" and 17" mbp (as well as the desktops) can support 2 monitors per thunderbolt port. allowing you to simply daisy chain two TB (thunderbolt) monitors together, or with a bit of a workaround(placing another TB device between the two monitors) , use a TB and DP monitor. You need at least one TB monitor for this to work though, and as you suspect, only apple makes these (i doubt though any other suppliers will EVER make a tb display, but could be wrong).
3) The rule with using mini dp device with thunderbolt devices is the DP device must be last in the chain always (which makes sense, since DP monitors dont have another port to plug anything else into anyway). are you saying you have a TB drive? either way, the drive probably has two TB ports, plug the drive in first, then take your DP dongle (be it dp-hdimi, dp-vga, dp-dvi whatever) and plug that into the external.
hope that all made sense