Where were Bnei Gad, Bnei Reuven and Chetzi Menasheh until now?
Malbim: Until now they were in their normal place among the Degalim. Now they passed in front of their brothers, like it says (Devarim 3:18).
What is "Chamushim"?
Rashi (Shemos 13:18, from Mechilta): It is armed.
Malbim: It is divided into troops of 50, with an officer of 50 over each troop. 1
In Shemos (13:18), Malbim agrees that it means armed. Rashi there brought another Perush that it means 'a fifth' (four fifths died during the plague of darkness). Here, it is feasible that the number who went to fight was a fifth of the total population of these tribes (including women and children)