In Settings > Profiles
Make a Quality Profile and select SDTV
Make 2 Release Profiles
1: must contain Horriblesubs
2: must contain 360p
Add a tag for both release profiles, eg. HSUBS
In the Series set the Quality Profile from above, and add the HSUBS tag
Now it should only grab Horriblesubs releases in 360p
Although this is semi-unnecessary as Horriblesubs typically only release in 360p or 480p but not both. So you could achieve the same with an SDTV Quality Profile and Release Profile 1 above. That way it will only download the Horriblesubs SD release. If you restrict it to 360p you won’t get a download at all for some series.