Is the SILK part of Opus compatible with the SILK implementation shipped in Skype?
No. The SILK codec, as submitted by Skype to the IETF was heavily modified as part of its integration within Opus. The modifications are significant enough that it is not possible to just write a "translator" and even sharing code between Opus and the "old SILK" would be highly non-trivial.
What is Opus Custom?
How do I use 44.1 kHz or some other sampling rate not supported by Opus?
What are the licensing requirements?
The Opus source code is released under the BSD license, which is a very permissive Open Source license. Commercial use and distribution (including in proprietary software) is permitted, provided that some conditions specified in the license are met.
Opus is also covered by some patents, which are available for free, under conditions that the authors believe are compatible with most (all?) open source licenses, including the GPL (v2 and v3).