In the future are you able to add a option to select a channel to replace thumbnails for that single channel only? Instead of having to manually input channel ID's? It be easier to have a one click solution for that.
This has now been added. Click the DeArrow logo in the top right of your browser and click "Channel specific settings" when you are on a video of someone's channel
If you set your profile to replace thumbnails, but set the default settings to replace nothing, it will only replace thumbnails for the channels in your list