After updating the OS and installing .NET Framework 3.5 (required by SQL Sever 2014 installation), I install SQL Server 2014, 2012, 2008R2, and 2008. SQL Server 2014 is the default instance. For reporting services, I use the default 'Install the Native Mode with Default Configuration' option. It turns out that they can co-exist on the same machine. The SSRS connections via SSMS, and the report manager web site work fine without any issue. Manual configurations with the Reporting Service Configuration Manager is not needed.
Maybe the only caveat is to use their own SSMS interface when you try to connect SSRS, that is, use SQL Server 2014 SSMS for SSRS 2014, SQL Server 2012 SSMS for SSRS 2012, and SQL Server 2008R2 SSMS for SSRS 2008R2 or SSRS 2008 (SQL Server 2008 and 2008R2 share the same interface). If you use a wrong interface, such as connecting to SSRS 2008R2 with SSMS 2014, you may get the invalid namespace error.
Below are the screenshot for the successful connections to SSRSs and Reporting Managers.