The idle animations are controlled in the actions.txt file, to add or remove animations all you need to do is to remove the animations that you do not want. When you open the actions.txt file, look for these lines (they're right near the top)
stand_staff 0 16777216 4
10.000000 staff_cstance_musket 0 59 1342177297 0 0.000000 0.000000 0.000000 0.000000
10.000000 staff_cstance_musket2 0 59 1342177297 0 0.000000 0.000000 0.000000 0.000000
10.000000 staff_cstance_musket3 0 59 1342177297 0 0.000000 0.000000 0.000000 0.000000
10.000000 staff_cstance_musket4 0 59 1342177297 0 0.000000 0.000000 0.000000 0.000000
stand_crossbow 0 16777216 4
10.000000 staff_cstance_musket 0 59 1342177297 0 0.000000 0.000000 0.000000 0.000000
10.000000 staff_cstance_musket2 0 59 1342177297 0 0.000000 0.000000 0.000000 0.000000
10.000000 staff_cstance_musket3 0 59 1342177297 0 0.000000 0.000000 0.000000 0.000000
10.000000 staff_cstance_musket4 0 59 1342177297 0 0.000000 0.000000 0.000000 0.000000
These animations are contained in the mmanimations.brf file in Resources. You don't need to do anything with the files in there, but it's useful for knowing which ones you want to remove and which ones you want to keep (name-to-animation referencing is what I'm talking about). Say you only want to keep idle animation 2, this is what you need to do:
1) Remove all other animation lines:
stand_staff 0 16777216 4
10.000000 staff_cstance_musket2 0 59 1342177297 0 0.000000 0.000000 0.000000 0.000000
stand_crossbow 0 16777216 4
10.000000 staff_cstance_musket2 0 59 1342177297 0 0.000000 0.000000 0.000000 0.000000
2) Change the number of animations that M&B will use from 4 to 1 (or however many you have decided to keep):
stand_staff 0 16777216 1
10.000000 staff_cstance_musket2 0 59 1342177297 0 0.000000 0.000000 0.000000 0.000000
stand_crossbow 0 16777216 1
10.000000 staff_cstance_musket2 0 59 1342177297 0 0.000000 0.000000 0.000000 0.000000
3) Change the number of seconds that the animation will last for from 10 seconds to 4 seconds
stand_staff 0 16777216 1
4.000000 staff_cstance_musket2 0 59 1342177297 0 0.000000 0.000000 0.000000 0.000000
stand_crossbow 0 16777216 1
4.000000 staff_cstance_musket2 0 59 1342177297 0 0.000000 0.000000 0.000000 0.000000
4) Save the file
^ This process also works in reverse, for adding animations as well