I'm posting this in case someone else runs into the same problem.
Had an issue when the fingers had collapsed in on themselves when adding the bone constraints. I don't know how, but the 'Original Length' value of the Stretch To constraint on the MCH-Fingers was higher than default. It doesn't look like it can be 0, but I set it as low as it will let me, which fixed the problem.
Hi Jake,
you just need to press the little x button on the right of where you have circled.
This will reset the value to be what it is meant to be.
This can happen if you add the constraint when things are not at their rest position, or if you add the constraint and then move things is edit mode.
That button will sort you out but you must go in the order of the hierarchy or you will introduce a stretching error down the chain.
So start at _01 the _02 and _03 etc.
(don't do _03 then _02 then _01 as you will just make it worse)
Hope that helps.