Here's a test i did in the beginning of strat5 to figure out how the donkeys work vs the others:
Bjarky's speed testing: Speedtest is done on Snow Planes (wich is 0.7 of the steppe/desert/planes speeds)
Animal speeds with 1 naked troop +2 animals & x crates:
Crate amount Donkey mpm Sumpter mpm Rouncey mpm No animal
0 57.915 41.828 57.915 32.175
1 49.228 35.553 49.228 12.87
2 41.798 35.553 41.798 12.87
4 16.052 20.503 16.052 12.87
7 12.87 12.87 12.87 12.87
I used crates since that was easyer for different purposes, but yeah the general rule applies here as well, 1goods = 1troop is the decent speed and if you have horses the same number of troops or above u get bonus speeds.
If your goods excede the troops number then you slow down alot, u can though use different types of goods and as long as each individuel goodstype is below the troops u wont get penalised as much.
Edit: Forgot to add that as long as you have more horses than goods it is no problem if you have more goods than troops, you will still retain the bonus speeds.