How much did you size up? I sized up 2 for my Loomstates which was perfect. But I'm nervous about 12.5 Oz. denim stretching more post-soak.My 501 STFs stretch so much that I have to size down 1 to get the best fit.
I have 8 pair of Gustin jeans, and each was within 1/2" of the spec size in the waist. Of course that represents a 1" difference between the biggest and the smallest. But I am happy with the fit in all of them.
I have Heavy Americans in 30 Straight and Oky Stds in 30 Slim. The HAs feel much softer than the Oky Stds in spite of the extra weight. Both fit the same in the top block, and both stretched about the same. So if you size up in the slim fit, you will get more room in the seat, but it might be too big in the waist. Choose your poison :)
I cuff most of mine. I never cuffed before getting into raw denim, but I got to appreciate the look, and I like to show off the selvedge. I did hem my Japan Black X Black and Grey Silks, and wear them both as pseudo dress pants.
No, no, 1000 times no. Each jean company's tag sizes are different. Measure your best fitting jeans, and compare to Gustin's Fit Guide. Assume the Gustin jeans will stretch 1" in the waist. This means they will feel too tight when they arrive, but will stretch out to become super comfortable after several days of wear.
Some people believe that vinegar will set the dye and reduce bleed. Levi's recommends vinegar for their 501 STFs for this reason. However, there was recently a discussion on this topic on Gustin's community board, and many people believe its a myth. Still, if you want to try it I don't think it will do any harm. I've never heard of vinegar being recommended as a softener. IMO, the best way to soften jeans is to just wear them :)
I have a number of Gustin jeans in both 30 straight and 30 slim. The top block on both straight and slim are nearly identical except for the lower rise on the slim. Although the slims sit lower on the hips, it's not enough of a difference to merit sizing up. You should go with 30 slim.