According to the SDK you say the After Effects API should be used in order to create Effects type of Plug In's for Adobe Premier.
We developed several AE plug in's (Effects) based on the CS6 SDK of AE and yet they don't show up / work on Premier Pro CS6.
Where can we find guidelines on how to create plug in's which work both on AE and Premier?
Is there a code sample for such Plug In?
Yes there are code samples. If you look at the Sample Project Descriptions in chp 1 of the SDK Guide (on page 35 in the CS6 SDK Guide), the ones that are Premiere Pro-compatible are marked as such in the description. More recent versions of the SDK will have a greater number of sample projects that are compatible with both hosts.
Also, you can refer to chp 10 of the SDK Guide on Premiere Pro, to read some general remarks on compatibility.
I will give it a try.