The way I see it....
The aim of his breeding is to lengthen the smooth bit at the top of the feather which will provide more colour. The feathers will overlap more at the visual point i.e. the tip, deepening the visual colour...well that's my theory.
So that short feather he is talking about is the bit at the top of the feather - the smooth bit. So I think you are looking at this the wrong way. He is not talking about the full feather length, per se.
When he writes "little short feather" I think it is all the same subject, I don't think he means there is not much short feather
And on hindsight (in light of my own explaination.) yes it would be Homozygous Intensive to Intensive to lengthen the "short feather" more.
But as I say that is just my interpretation (wish I knew Spanish !) I could be wrong.....
Thr avatar is one of the few
Cock Lizards I bred last year, it is a cock, the photo is before 1st moult.
The hard drive with most of my canary pictures on that I took before my DSLR died is kaputt, so this about the only picture I have because I put it on another machine as the desktop.