Is it ever appropriate to mix up "I" and "one" in the same sentence?

In my last question on English L & U SE, I was strongly tempted to write the following:

Every so often I've thought I've chanced across most of them [literary Biblical phrases], but as one keeps reading, one keeps finding more.

but wrote instead:

Every so often I've thought I've chanced across most of them , but as I keep reading, I keep finding more.

My thinking, in preferring the former over the latter, was that while both sentences relate that the first observation is personal, that is, that it was I who thought he'd come across most of these phrases, the latter sentence better emphasized that the second observation is universal; that is, anyone who reads a fair bit will continually discover these phrases in their reading.

Unfortunately, being on a site with an exacting user base caused my inner matronly grammarian to kick in and I changed the sentence, based on some concern I was inappropriately mixing the grammatical persons. As well, I vaguely remember a not insignificant minority of questions on the SAT II Writing as being a stickler for this sort of thing when I took it some years ago. On the other hand, I'm not sure if this isn't perhaps another one of those 6th-grade kludges teachers tell you because either a) the rule is too complicated or b) it's not actually a rule at all, but a myth.

Anyway, sorry for the long-winded background -- can anyone help me and give me a general usage rule, as well as information on how strictly it is adhered to in literary and professional writing?

"I" - Pronoun, First person singular.

"One" - Pronoun, Third person singular.

So the use of I in the sentences above implies that you specifically are finding more such phrases. Whereas the use of "one" implies that both yourself and people in general would find such phrases as they kept reading.

"One" and "I" may be mixed up to denote the third and first person.

For example:

"One should say that Geoff had difficulties with his weight, but I always just said he was fat."

I think you've got it exactly right. You're only asserting that it is YOU who have "chanced across most of them", but it is a universal expression that "as one keeps reading, one keeps finding more."

To settle on one or the other is to reduce the amount of information in the message, plain and simple.