English Language (?)

 I have a bad habit of analyzing the English language and trying to figure out why things are the way they are. Here are a couple examples:

1) Why is collocation a valid way of spelling co-location? If 'co' is the prefix to location, why do we get an extra 'l'? Cohhabitant isn't valid is it?

2) Is is try that sometimes we use 'an' in front of acronyms instead of 'a'? Is there an English rule that we treat non-vowel starting acronyms as if they were vowels? For example, when I say "I want to buy a RRSP" I think I use 'an'. Am I correct?

 Just something to take your thoughts away from work.