I have done a little fusing with unknown COE glass and from what I have read you use glass only from the same piece. That is what I did with mine. My reading tells me that a crack can happen down the road, not only immediately, so I didn't want to chance it.
I don't know about the polariscope and showing stress, but as far as I know, once you mix COEs there is no way to fix the piece if it stresses.
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