Chuckie, you're not right :) .
The speed range is 99, not 50.
Sometimes 105 still works but
The less number, the more combo chance.
99 is 100% for combo. It's called "speed chain" in some guides.

But in my case, I was right.
All speed of each attacker must be greater than speed of the target.
One of my pets has the speed less than the target's. So they can't combo.
For being sure, I tested with another monster that has less speed (200)
My slowest pet has speed 270 (>100), and my team got combo.

The new thing's for you. :)