Apparently only very old SIM cards work with the Super SIM method.
Since the providers you named are pretty new, I'd say: forget about it.

I have a very very old T-Mobile SIM card (15 years or so) which I'll try this weekend but otherwise, I think chances are pretty slim to find a (newer) SIM card in Germany which works with the Super SIM method.

Regarding international roaming with an AT&T card: you need to wait 90 days as far as I remember, not 6 months until international roaming is activated.
You can speed up this process by leaving a "virtual deposit" of up to 1000 USD on your credit card. Meaning: AT&T "blocks" 1000 USD on your credit card, being able to cash it in when they want but they don't really take the money, it works the same way car rentals and/or better hotels operate. You just need to go to a local AT&T corporate shop and talk to these guys and of course you need a valid credit card with a larger "limit".