Knowing Your Downshifting Limits.

I like to think that I can drive a manual pretty well. However, there's always something I was never sure about.

How do you know what gear you can downshift to?
Let's say you're doing 55mph in 6th gear and you want to take off. Would it be safe to downshift to second?
I'm not sure if my concept is right, but since my second gear tops out at (I think) 60mph, it should be okay, right? It shouldn't over-rev?

Also, if you ARE going to downshift just to get more torque, should you go in sequence? As in...shift to 5th, then 4th, then 3rd, then 2nd? Or is it okay to just shift from 6th, straight to 2nd?

If you're going 55, and your 2nd gear redlined at 60, it would make no sense to downshift to that gear, as you'll be there for 1/2 second, then you'll just have to shift to 3rd

For me, it just took habit. I learned that from "x" speed to "x" speed, that I should be in this gear, and if I want to go for it, it is safe to be in this gear. I also learned speed according to RPMs, like 2700 revs in 4th is about 55-60.

And no, you don't have to go in sequence