Not just the speed will be a factor of the trooper. And actually, if you starting to made a trooper class then the speed of the others isn't really a factor at all - since it would be the mainline armor of the units among their rank, everyone else should keep pace with them rather than they needs to keep pace with the others.
Honestly, I'd think that if you really want a trooper then it needs to be cheap, so put a 3/5 mech on the lesser tonnage than 100 would gives you the better trooper, for it would be cheap and durable consider the cost at least.
If you're going to argue that a Highlander or a Stalker is a trooper as a 3/5 assault 'Mech, then the Atlas is a trooper 'Mech by the same token, at which point the term becomes essentially meaningless. After all, an Atlas is essentially a Highlander that trades jump jets for heat sinks and a little more armor.
Traditionally, a "trooper" has been used to describe, for the Inner Sphere at least, a 4/6 general-purpose medium or heavy 'Mech. Thugs and BattleMasters can be extensions of that, since the only difference between the 75/80/85 tonner at that point is maximum armor. A 3/5 'Mech gets used differently.