Thank you all for your great advice and information! And thank you for keeping the topic going

.
I haven't narrowed down my options, but I did change them around a bit to the Harz Roller, Waterslager, and Russian Singer (although the Russian Singer seems very very high pitched).
I'm actually leaning most towards the Roller at this point since they seem like the best bet for gentle singing, but I'm still unsure because I would miss out on some of the nice flute sounds that the other canaries make. The Waterslager seems to have a nice combination of Harz Roller sounds and some flutier sounds, but their song just seems a bit too intense/forceful for me to take!
I will listen to some Gloster songs as well

. Thanks again all!