Alright, I'm still working through RTK1, so I'm not that concerned about readings yet, but I just noticed something odd. Most resources list イツ as one of the readings for 「一」 (one), and I found an example in 同一 (どういつ). But in RTK2 Heisig uses イッ (notice little tsu) instead.

So, can a つ reading change into っ in some contexts? How does that work? Because they seem rather different to me. Also 十 does have a reading ジッ, so it's not like 「ッ」is avoided in dictionaries.

イッ is an abbreviation of いち, not いつ. For example, you would say 一ヶ月 いっかげつ because it's easier to say than いちかげつ. いつ seems to be used at the end of 熟語 and is fairly rare, but since you'll end up learning readings through vocabulary, knowing when to use which one shouldn't be much of a problem.

So then why does Heisig present イッ as an onyomi in frame 966 (RTK2). Or am I misinterpreting it?

It's a bit odd, that. Heisig's example compound in 966 is 一緒 which I would have put in the same boat as 一ヶ月 as Javizy says. I'd just ignore it if I were you.

Oh the wonders of RTK2

Looking at the index, frame 966 is listed under イツ, not イッ. Guess I'll file this one under mistake.