RD + cassette compatability I have a Shimano 11-42T 10s cassette and Shimano XT M786 SGS Deore RD, can I make these work together by using goatlink or am I wasting my time?
The RD-M786-SGS is rated for a big sprocket of max 36T, so if you want to follow strictly Shimano specs, no.
That being said, Shimano has high standards and is very conservative, so it is possible that it works. I personally just installed on my bike a Microshift 9sp 11-42T cassette (no goatlink) on an Acera RD-M3000, rated for 36T max (I had to adjust the B-screw). If you want to do things properly, you may also need to add some links to the chain, especially in a 1x setup. I have a 2x (46/30), I've let the chain as is and try to avoid even more the big chainring/big sprocket combination — I was also fearing that adding some links would cause the chain to swing too much on rough surfaces.
It seems to work well so far, but I haven't tested extensively in rough terrains. That being said, I tried because I have found several comments of other persons saying they have installed successfully this cassette on derailleurs rated for 36T max (including some old Deore and XTs).
I would assume a similar reasoning is also valid in your case.