The problem with mixing Inf and Militia is that you lose 25% of your movement speed.
That said, I've found big divisions of Militia are extremely useful to fill the line for any countries that don't have the IC to just throw infantry in there. Garrison are good in small divisions so the AI will move useful divisions to the front, but that's about it. Military police are useless because nationalism creates a minimum revolt risk that is exactly the same as the nationalism modifier, so unless you have lots of dissent, suppression isn't useful.
That said, I've found big divisions of Militia are extremely useful to fill the line for any countries that don't have the IC to just throw infantry in there. Garrison are good in small divisions so the AI will move useful divisions to the front, but that's about it. Military police are useless because nationalism creates a minimum revolt risk that is exactly the same as the nationalism modifier, so unless you have lots of dissent, suppression isn't useful.