Glad to see this post. I agree that they can look similar!
My mom is an E1 and before I had her take the test, I thought she was a phobic 6. She has always had a lot of anxiety.
Now, looking back I see where I typed her wrong and what you say about perfectionism plays into that. Also, the fact I *saw* anxiety is a clue that she probably wasn't a 6.
Something I ask in Enneagram typing (if I'm doing it myself) is: What enneagram motivation DON'T I see in the forefront of this person?
I saw her anxiety, I didn't see a lot of anger from her.
I don't know a lot about 1's but I think what they have in common with 6's is kind of a focus on control.
I see that control -of anger/extremes - in 1's perfectionism.
6's in control of - fear - in anticipating it (CP) or implementing safeguards (phobic) against it.
I have so many other anecdotal similarities I see: I won't say this is for all 1's or 6's but this is what I see between my mom and myself:
She uses hierarchal structure because it defines what is appropriate. She knows what is expected. I think she measures herself against these things internally. Whether she buys in or not, I'm not sure. BUT when she does?? She does with passion and conviction. The reformer/crusader comes into play here.
I think 1's are looking FOR something to put their "inappropriate" (self-deemed) feelings and channel that into "appropriate" outlets. Once they have the outlet, they feel free, more at peace.
(She's also ISTJ so...overlap here, I'm sure)
I use hierarchal structure to question myself and my beliefs, it's a jumping off point. Should I buy in? Should I not? How much? It's always oscillating. I'm NEVER going to buy in fully. Not like a 1 does, at least because the very nature of my motivation prevents full committment

I'm just happy it's there.