Can we use millet (of different varieties) instead of rice during Ekadasi, Dwadasi, and Ammavasyai? If so, what are the millets that can be used on Ekadasi, and how to break them finely?

Only rice has to be broken when it is used during Ekadasi, millets can be taken without breaking it.
However, it appears that millet cannot be used primarily during Dwadasi or Ammavasyai as we partake cooked rice. If millets are used for diabetes, a morsel of cooked rice should be used for doing pariseshanam and then one can consume cooked millets as a substitute for rice.
Ragi, horsetail millet, little millets can be used in millets.