A vs. an comparison is not really fair, as it's very natural to use these articles in front of most nouns, thus you expect to hear them and separate them from main nouns they modify. Russians don't have these small words in between main words and therefore anything in between would be considered a part of that word. For example, if you said something like "пошли ко горке", my first thought would be: "What the hell is огорка?" That's when coming from a native though, when speaking to a foreigner you'd expect these kinds of things and probably figure out what they meant.

As for ко, it's not that widespread, so it's hard to figure out its unusual uses.