Most field MP units deploy on a 1:1 ratio and are going to be on a 1:2 soon I hear. Right now after you deploy 3 times you are allowed to get out of the field unit and go work baseline to get some base mp work in.
You dont HAVE to have a secondary MOS, I am a Non Lethal Weapons Instructor and its currently in the works as being worked into as a secondary MOS like the other branches. It is a competition to be k9 but just realize if you want it bad enough you will get it. Most Marines compete for this in their MP school to get sent straight there and the one that gets the best grades and writes a good paper will end up going. You can also go after you get stationed somewhere if they have school seats available.