Hey Red.
The answer to all of your questions (except number 3) is Yes, with a few buts.......
1a. Yes..............But.......... you need to be careful with the impedance of your speakers. You must make sure you match the impedance of the speakers to the correct impedance output of the amp. If I'm right your HotRod Dlx has a single 12 8ohm speaker it in it. I believe that the TIny Terror has a pair of 8ohm outputs. Just plug the HotRod speaker cable into either one of the TT 8ohm outputs.
You might find that the HotRod speaker is hardwired through the amp chassis and so may not be able to 'unplug' it. In this case you'd need to cut the speaker cable and fit a jack and socket arrangement so that you can plug/unplug away.
DONT DO ANY MODIFICATIONS TO YOUR AMP UNLESS YOU ARE TOTALLY CONFIDENT IN WHAT YOU ARE DOING. THESE THINGS CAN KILL YOU. ITS ALL AT YOUR OWN RISK. GET A COMPETANT TECH TO DO IT IF YOU ARE UNSURE........and all the other usual warnings and disclaimers.
1b. Yes.......There is a unit available on the market which does amp switching to a single speaker cab. Never tried it myself, but it's supposed to work quite well. It's made by Radial and it's called the Headbone. Wouldn't like to make one of these myself, as tube amps tend to get very unhappy if they are not connected to a load (speaker) properly. This box has s series of switching relays and resistors to make sure that the amps get loaded properly and all in the correct order.
http://www.tonebone.com/tb-headbone-vt.htm
2. Yes.........Assuming the amp has an EXT speaker output. (I've jsut checked the spec on the Fender website. assuming your's is up to current spec, then you are cool) Just make sure your use a 8ohm cabinet.
3. Possibly. Dont know the implications of running the headbone into a multi-cabinet arrangement. It would probably be OK so long as you keep the total impedance of the load all at 8ohms.
I think really you need to do a bit of research of speakers and impedances and impedance matching to make sure you fully understand whats going on. It's not terribly difficult, but there are lots of different ways of configuring/wiring speakers and each of these can affect the impedance of the system. As I mentioned earlier tube amps tend to get quite unhappy if the are connected to an improper load. At best you'll might blow a tube or two (these are cheap and easy(ish) to replace), at worst you could blow the entire output section of your amp (including the transformer, which is generally the expensive bit)
Check out Celestion's website for a good explanation of loudspeaker impedances.
http://professional.celestion.com/guitar/features/drdecibel/index.asp
look at the two drop down menus and you should see some options for speaker impedances, and wiring diagrams.
Ask more questions here of you get stuck....
Good luck
