If anyone is familiar with this particular amp, do you think it's powerful enough to drive 2 X 12" speakers?
Yes it is. And more... The standard test for sensitivity of any speaker is 1W!
As the AD30 has a solid state power amp, you could even up the impedance of the cab to 16 ohm without fear of damaging anything (solid state tolerates even an open circuit, which would destroy a valve amp) - you just lose half the power output.
When it comes to the speaker wiring, what is the difference between wiring the speakers in parallel and wiring in serial?
The main thing (aside from impedance which you have covered) is
failstates: what happens if one of a pair (or quad, whatever) of speakers blows and goes open circuit (much more likely than a short circuit with speakers)? With series wiring, both speakers stop working (the second speaker isn't blown, it just gets no signal and will work if the blown speaker is bypassed) and impedance jumps to infinite (once again, not a problem with SS amp). Parallel wiring, the second speaker continues working, although the impedance the amp sees doubles. Pretty minor either way with a solid state amp, but the parallel wiring at least lets you finish the song/gig.
Could I wire it so that the internal 10" and external 2 X 12" cabinet would work together? What would I have to watch out for if I did that?
Not worth it IMO. You'd have to rig it so the extension speaker jack wires the external in series with the internal to keep the impedance above 8 Ohm - and lose power every time you do so. Much better to fit a 16 Ohm speaker to the amp - which will halve the amp's power when used alone - and then have a 16 ohm extension cab that together with the internal speaker brings the total down to 8 ohm for full power.
