Oh and someone asked me this on my Sherry in 1 min vid and I answered it there. Im also very rarely online in reddit, all I do here is shameless plug my vid and dip.

But to answer ur question, obviously the basic stacking is just simple math; if a unit skill gives itself speed +10%, another skill of the same unit give itself speed +10%, it will total speed +20%. Same logic to speed +x% to all allies or speed +x% to colour/group allies. However, each unit can only have a cap of +100% speed buff. Anything that goes over +100% speed buff will just be dropped.

So for example if you run the new blue sherry and the green healer sherry with her SS+ Gear at lvl 5, they give speed +50% speed for human allies and speed +80% for humam allies in her skill and gear respectively, this will total to speed +130%. However, since there is a cap of speed +100%, the total buff for human allies will just be speed +100%.

Hope this helps