combining stable and testing in sources.list

[murtaugh@stat.orst.edu]

I mistakenly added two lines with 'testing' to my 'sources.list' file, which now looks like this:

deb http://debian.osuosl.org/debian/ stretch main non-free contrib
deb-src http://debian.osuosl.org/debian/ stretch main non-free contrib

deb http://security.debian.org/debian-security stretch/updates main contrib non-free
deb-src http://security.debian.org/debian-security stretch/updates main contrib non-free

deb http://debian.osuosl.org/debian/ stretch-updates main contrib non-free
deb-src http://debian.osuosl.org/debian/ stretch-updates main contrib non-free

# my bad:
deb http://ftp.us.debian.org/debian/ testing main non-free contrib
deb-src http://ftp.us.debian.org/debian/ testing main non-free contrib

I have read that it's difficult or impossible to revert to 'stable' once 'testing' packages have been installed. I have also read that it's not a good idea to have both 'stable' and 'testing' repositories in 'sources.list', so I'm wondering how to proceed.  I experimented with replacing 'stretch' by 'buster' in the first six lines above (and removing the last two lines), but then the system couldn't find the stable google-chrome repository in '/etc/apt/sources.list.d'.  I now have the uncomfortable feeling that I'm playing with fire.

Thanks in advance for any suggestions about if/how I should edit my 'sources.list' file!

-Paul