What level should I start the reverse course at(AKA, german to english)?
Don't base it on level. Base it on your overall proficiency in German. The reverse tree is much harder because you are translating into the target language. I'm on level 19 and have done a few of the reverse tree exercises, but have stopped for the time being because there are grammatical things I haven't learned yet to complete them effectively.
It doesn't really matter when you start because there's no requirement to go on. So if you find it begins to get too hard than just switch back to German from English. I don't think it affects your streak but then I've never done it but I assume so cause there's people with huge streaks and how many languages I can not count so it must be.