I have a question on a single character menmonic and it seemed not enough to start a thread, so I thought I could give a broader subject in case other people had similar questions.

In the kanji "Waterfall" (http://kanji.koohii.com/study/kanji/waterfall) Heisig explicitly states not to use a "Dragon" as a primitive, but to split the right part of the kanji as "Vase + Eel".

I find myself a lot more comfortable using "Dragon" as a primitive than "Vase + Eel" (I had to repeat the kanji a few times because using "Vase + Eel" didn't work out for me... Now, 100 frames later, I remember it without using any story as the "Heisig doesn't want me to use 'Dragon' primitive" kanji).

Why does he say not to use "Dragon" as a primitive?
Do we encounter a "Dragon" primitive later on and it's to avoid mixing them up?

There's an old form of the dragon kanji, 龍, that appears at the end of the book and which is used in many more kanji than the simplified form 竜. This is because the japanese script reform was as inconsistent as the rest of the language. Maybe Heisig didn't want you to mix both primitives to avoid recalling the wrong one later.

Thank you!
It's more or less what I was expecting, but for other primitives he told explicitly "we'll see later another primitive with this meaning, so don't use it now". Taking a look at the end of RTK1 I see that old-dragon is used here: http://kanji.koohii.com/study/kanji/2025 but in the original Heisig description the primitive is broken up in pieces.

The old-dragon itself seems to be introduced only in RTK3: http://kanji.koohii.com/study/kanji/2981 which could be the reason Heisig didn't say "We'll see another kanji meaning dragon later, and use it as a primitive": 龍 is in another book.

Now that I know what the problem was I'll still use 竜 as a primitive but with a keyword of "Young Dragon" and will use "Old Dragon" for 龍 when I get to it.

Ephel Wrote:The old-dragon itself seems to be introduced only in RTK3: http://kanji.koohii.com/study/kanji/2981 which could be the reason Heisig didn't say "We'll see another kanji meaning dragon later, and use it as a primitive": 龍 is in another book.

It's now in RTK1, because there have been extra kanji added to the official list.

I used to live in a "city" in Ibaraki prefecture that was called Ryugasaki, they used both spelling. 龍ヶ崎 and 竜ヶ崎, sometimes you even see 竜ケ崎. Japanese! (Slaps forehand)
I think it came from the merging of the towns in the early 90's. It seems that bigger 龍ヶ崎annexed the surrounding smaller towns and became 竜ヶ崎, but you'll see both 龍 and 竜 used in the same official documents.