Buckeye Rose

I am in process of making a memory quilt of clothes from my brother-in-law, who passed recently and I have a couple questions. There is a variety of different fabrics and thicknesses (t-shirst, plaid shorts, jackets, robes, etc.). 1. Should I add interfacing to all pieces, or just to the knit fabrics? In order to have enough 10-1/2" blocks, I am taking the fabrics from two similar t-shirts and making a 4-patch. 2. I am unsure if I should join the squares and then add the interfacing? Or add the interfacing first? I am afraid if I add the interfacing first that it will be too stiff at the seams. 3. What is the best fusible interfacing - one that will be soft after washing so the quilt is cuddly and comforting?

Thanks much (in advance...LOL) for all your help. Rose