You wouldn't have to remove them to take cuttings. The cuttings should be about 3" long or so when you cut them. If you have the tubers in the boxes with their necks sticking out of the mix the sprouts are easy to get at. You only need to have about half the tuber buried.
Just be sure you have the tubers labeled & that you then label the pot you put the cutting in.
We use a potting mix to start so I'm not sure about your mix but as long as it's not too dry it should be OK I think.
The temp is good.
Look at the Colorado Dahlia Society site for tips on cutting- this is the page.

Here is a link that might be useful: Colorado Dahlia Society