> Knee-high, flat-ish riding style boots? Or the scrunchy ones?
Yup, either one would work, and the ones you posted a link to. They would just give your look a different vibe.
> And if you were wearing with dark blue skinnies/straight legs, would you go > for black or brown/cognac ?
For me, it would depend on what other colors I was wearing. But as loulou mentioned, dark blue would go with black and brown.
> One last question - if you found that you couldn't get skinnies to look right on > you, would straight legs still work for tucking into boots?
Yes, you can, depending on the straight legs and the jeans. They'd work better with slouchy boots, since you don't need to fuss with the hem so much.