Finally got a commerce speed thing. So this time I decided to try to measure animation speeds because I really don't feel like the other thing.
Measured the walking cycle of an elephant.
Nothing: (121-164), 0.716 seconds.
Fleet Feet: (139-178), 0.650 seconds.
Alpaca Robe: (109-145), 0.600 seconds.
Commerce 17: (125-153), 0.466 seconds.
17+Fleet: (079-104), 0.4166 seconds.
17+Robe: (115-145), 0.500 seconds.
17+Fleet+Robe: (162-188), 0.433 seconds.
Note that this is rough, it's not granular enough for precise measurements but it shows that those things all stack.
There's definitely a movement speed cap. I tried stacking a lot of movement speed buffs and I stopped getting faster. March Song, if it did anything, was negligible, and Shadow Cloak gave me no discernable speed boost at all.