A good news is that, we already transferred our iOSEdition to the Unified API few weeks ago, and successfully submitted our first 64bit iOS project to the app store. So it will be very close to make it release (I think it should be ready around the first week of March). We planed to have two assemblies then. One to maintain the old one, while another one for the Unified API. The new APIs is a little different. For example, we used Frame property on UIView a lot. It is used to be RectangleF; while in the Unified API, it becomes CGRect. So we might need to review all our existing samples and GettingStart guide and make sure all works fine with it.
For the Xamarin starter plan, I think we might not support it right now. Because Xamarin limits the size of the compiled user code (IL) to 64k (It is double size now. It is used to be 32k). xamarin.com/faq#q18
And our dll is far more than that limit. It is not easy to make the assembly smaller. I will talk to our manager later and see if there is a chance to split the functionality and make the assemblies smaller or build a simplified version.
We didn't test it using the VS Community, I think it should support the Xamarin plugin for Vistual Stuido. We will test it later and let you know.