Mango's free version enables up to 20 data points. You would probably also not want to use the NoSql module in a free system.

I haven't personally tried to connect to a NEST device, but some searching makes it look like it would take some effort. It may be possible with the HTTP Retriever, but if not it may require either a Firebase4J integration or some interesting solution utilizing another API: https://www.firebase.com/docs/rest/quickstart.html

Belkin's API doesn't seem too readily available... I didn't sign up to enter their documentation. There is a 3rd party Python implementation, it appears.

You are that man! It asked for my password after I enetered "sudo ./ma.sh start"...

Another fine JOB JOEL...

I'm not sure where you heard that Mango was for residential. Mango has always been a commercial product and continues to be used in a wide variety of industries around the globe. I'm not sure if anyone here is familiar with the other systems you mentioned so maybe you can give Mango a try and report back on what you think the differences are.