How to account for mobile devices that are not company owned

Answer: BYOD physical devices are typically excluded from the ISMS scope because you cannot control them completely, but you should include in the ISMS scope the company data on those devices - in that case, you simply list those data in your asset list and in your risk assessment.

This article might help you: How to write an easy-to-use BYOD policy compliant with ISO 27001 https://advisera.com/27001academy/blog/2015/09/07/how-to-write-an-easy-to-use-byod-policy-compliant-with-iso-27001/

We have interfaces setup with our clients. W ould we consider those interfaces to be a separate asset, or would we only account for the data when it is stored in a database?

Answer: I'm not sure what do you mean by interfaces - if you refer to some devices or software, then you should include those assets in your asset list. In some cases you will view data separately from the devices - e.g. you will list a database separately from a physical server, in other cases you can view server as a both physical server and data on this server - you are free to do it any way you feel more appropriate.