Do all A and B band G2 or G3 wireless work well together?

An 'A' frequency block would not be interchangeable with a 'B' frequency block, but usable at the same time... as a 'second system'. There would be no difference sonically. The G2 and G3 would be interchangeable in the same frequency block.. though the G3 has a wider RF bandwidth. Our resident Sennheiser expert John will most likely confirm and/or elaborate.

Do all A and B band G2 or G3 wireless work well together?

An 'A' frequency block would not be interchangeable with a 'B' frequency block, but usable at the same time... as a 'second system'. There would be no difference sonically. The G2 and G3 would be interchangeable in the same frequency block.. though the G3 has a wider RF bandwidth. Our resident Sennheiser expert John will most likely confirm and/or elaborate.

Yes - this is basically correct.

In any band all the frequencies in the same block would be IM-free but you must not use any frequency from another block.

Yes you can use systems from band A and band B together, but I would talk to Sennheiser USA as to which blocks in each band are best to use together.

I own a G2 from Block B. You seem to imply that staying within Block B with a G2 or G3 is the best choice for my second system.

I own a G2 from Block B. You seem to imply that staying within Block B with a G2 or G3 is the best choice for my second system.

If it's only two systems it doesn't really matter - and may be an advantage to gave a different band if you move around a lot.

However, for a multi-channel system it's best to plan it out and have it as a system in the same band.

I have a G2 handheld mic and G2 receiver. I want the option to use a wireless boompole with a shotgun mic instead of the handheld. Can I use a block B G3 plug-on with my block B G2 receiver?

Yes, I do this all the time.

I have a G2 handheld mic and G2 receiver. I want the option to use a wireless boompole with a shotgun mic instead of the handheld. Can I use a block B G3 plug-on with my block B G2 receiver?

Yes, although the G3 has a larger frequency window. Just make sure it's tuned to the same frequency.

But, be aware, there is no phantom power on the SKP 100 G3 (nor on the G2 /G1) - if you need phantom power you will need the SKP 2000 (which replaced the SKP 500 G2).

Do I assume correctly that a G2 sk500 bodypack will work (with restricted functionality) with a G2 sk100 bodypack receiver in the same band?

Yes - stay within the same band and G2 and 3 kit is not a problem - and even the old first generation products are fine to, if the receiver has the tone lock turned off. Good on Sennheiser for keeping the actual basic specs alone between generations.