You are thinking along the right lines. I run a GTB2 in my oval pan car, running a 1s lipo. I've used both a receiver pack and a novak 1s booster.

You are correct that the speedo will be powered by the battery pack. For the rest of your gear, the speedo is sending current out thru it's bec circuit on the red wire to the receiver. Since you are using a 1s pack, the most it can send out is 3.7v. Not enough for receivers, servos, and a transponder.

Both the receiver pack and booster work equally well. I've got one of each in my two separate cars. What I've found works best is to:

1) Switch everything off
2) Turn on the radio
3) Turn on the booster or receiver pack
4) Switch on the speedo for a couple seconds, then switch it back off

You should be good to go.

The alternate procedure is to remove the red wire from the speedos receiver harness. In this case, Step 4 will become "switch the speedo on and leave it on".

Either way, the speedo is no longer powering the receiver now (it's OFF or disconnected). So you can now either plug a receiver pack into the receiver (into an open channel or the BEC jack) and power your electronics that way. OR, you can hook a booster up to the battery leads (at the battery connector or ESC side - both work). Then plug the booster into the receiver (an open channel or the BEC jack). This will take power from the pack, correct the voltage, and supply it to the receiver.