I wrote a post about my switch to colemak the other day and how i'm practicing addresses some of your concerns: http://gibrown.com/2015/05/18/colemak-0-to-40-wpm-in-40-hour...

TLDR: build frequent wordlists and lists of n-grams for practicing. Here's mine with lists for English, Unix Commands, and PHP code: https://github.com/gibrown/learn-colemak