Pretty much personal preference and I'm sure you'll see recommendations for all of them after a while. I've used a lot of different plastics over the years but pretty much only use MF or Bait Plastics (polysol) now.
It's really something you have to experiment with since everyone has their own idea of how much scent is the right amount and similar scents from different sources will have different potency.
You won't have an issue adding MF scents into your plastic, same with Pro Cure or smelly jelly. Generally when I pour, I get my plastic up to 350, then add colorant. Once it's cooled to 325 - 330 or so I add the flake and scent and shoot.
If you plan on using a mixer you won't be able to extend the thermocouple down into the pot. On mine, I built a bracket from leg to leg diagonally across the bottom of the pot and have the tip of the thermocouple contact the center of the pot (under a little tension). I made a little divot with a drill dead center for the tip of the thermocouple to sit into.
If I'm shooting a bunch of baits I'll throw them in the water straight out of the mold to cool them down, but every 15 minutes or so I take them all out of the water and lay them out to dry on a towel. It makes it so you don't have to mess around with laying them straight right out of the mold when they're still really soft.
Every now and then I see that there's a new topic in the cookbook sticky so I open it up to see what's new. It seems like a lot of folks are going in there asking how to make a color and that ends up with a lot of messages back and forth. I would suggest that if you need help with a color that isn't in the cookbook that you ask in the regular part of the soft plastic forum and once you get the color dialed in you post the recipe in the sticky cookbook thread.
Just my opinion though
I think the ratio depends on how concentrated you want the finished colorant to be. You can tell some companies add less powder per oz of oil and some add more.
I could be completely off base though
Found the thread from last year... His website isn't working but he has a facebook presence.
http://www.tackleunderground.com/community/topic/35252-who-sells-calhouns-plastic/?tab=comments#comment-290061