I'm considering getting a Canon 1.4X TC II, but am concerned about compatibility.

It only works with certain Canon lenses. I assume because of clearance issues with the rear element, yes?

I also figure that, since any alt lens mounted with an EOS adapter will have MORE clearance than any Canon lens, they should all pretty much work with the TC. Is this correct?

I don't think this TC works with lenses other than Canon telephotos above 300mm.
I could be wrong about that , but the 1.4x II does have a protruding front element that won't allow it to be used with our alt glass. At least the shorter ones, maybe telephotos are different.

JayT

Worked great on my Tamron 400/4. Works great on my Angenieux 180/2.3 and Nikon 300/4.5 AIS (non ED).

Thanks, Paul.

The Nikon 300/4.5 AIS is one of my main concerns. (great lens.)

A Kenko Pro 1.4 TC will work with almost anything and is quite good too.

I have put a 1.4X on my 70-200f4L with no problems so it will work with other Canon lenses besides just 300mm+ primes.

Non-Canon TCs in EF mount fit more lenses. Might be a better choice.

Edit: Actually reading your post I see you know that. I like Canon with Canon, but I would get the Kenko to use with alts.