SSRS 2016 on SQL Server 2014 I have question about using SSRS 2016 on SQL server 2014 

First of all, I don't have much experience with Reporting services.

Company use SQL server 2014 and they have SQL database on it (DB is around 100GB). I could install SSRS 2014 on this server (because they have licensed SQL 2014), but I want to use SSRS 2016, because SSRS 2016 has many new features (web portal, mobile reports, custom design, KPI,..).

My idea is to install SQL 2016 Express version (It's free - 10GB) to install SSRS 2016 on it, only to store this two meta ReportServer DB and then connect to this production SQL 2014 DB to create queries on it. 

Is this acceptable solution? Are there any other possibilities or they must buy SQL 2016 license to upgrade their SQL?

Thank you and best regards
SSRS Express will only connect to its own SQL Server instance as a data source. You can’t use it in the way you’re describing it. Sorry.