Ok, several corrections, based on testing I did tonight.
First, you do not gain Speed Rating equal to 20% of your highest rating stat. Not only does it not actually show as rating in the movement speed tooltip, but at 120, you get 1% speed for only 20 speed rating initially (though it appears to now have a diminishing returns curve to it. ex. my monk gets 2.80% movement from 62 rating and 10.14% from 312 rating. My warrior gets 2.17% from 47 rating).
On my shaman, a 370 Longstrider grants 18% of highest, up to 6%. I get 3% from it. My highest rating is Crit at 1040 rating, and my highest percentage is also Crit at 19.44%, and he has no speed tertiary stat anywhere on his gear. Now, the tooltip for speed is only precise to the whole integer level, so the 3% displayed could be anywhere between 2.5% and 3.49%.
18% of my 19.44% crit is 3.4992%. 18% of 1040 rating is 187.2, which would be 9.36% movement speed before diminishing returns. Considering that 62 rating was worth 2.8% on my monk, I doubt that 187 rating would be worth 3.49% or less.
So the math of the speed appears to line up with granting raw speed percentage based on your highest character sheet stat percentage, with the exception of mastery. For mastery, it appears to scale off mastery points*. For my shaman, while he has 26.33% mastery, that only represents 14.04 mastery points, which is less than my 19.44% in crit.
Conclusion: Longstrider is definitively not granting speed rating as a percentage of your highest secondary stat rating. It appears to grant raw movement speed percentage as a fraction of your actual highest percentage on your character sheet (with special rules for mastery).
Other than that, it also stacks (additively) with all other movement speed increasing effects that alter unmounted movement speed. It does not affect mounted movement speed, and thus obviously does not stack with mounted movement speed bonuses like the Heart of the Crusader paladin passive.
*You get 1 mastery point for every 72 mastery rating, regardless of class or spec. Each point of mastery gets converted into the percentage on your character sheet based on your base (naked) mastery percentage. The conversion ratio for your spec is the amount of mastery percentage you have naked divided by 8, because everyone has 8 mastery points baseline. For example, my elemental shaman has 15% character sheet mastery percentage naked, which is 15 / 8 = 1.875% per mastery point. He has 435 mastery rating, which grants 435 / 72 = 6.042 mastery points. That grants an additional 6.042 * 1.875% = 11.328% mastery. And sure enough, his mastery tooltip shows Mastery: 435 [+11.33%], and his total mastery is 26.33% (15% + 11.33%).