I got some infos online, and I think it would be worth to share it here, so basically this post is becoming a digital post-it[/edit]
Well, seems I was wrong, at least according to Wikipedia http://en.wikipedia.org/wiki/Microchip_implant_(animal)
Numerous references in print state that the incompatibilities between different chip types are a matter of "frequency". One may find claims that early ISO adopters in the US endangered their customers' pets by giving them ISO chips that work at a "different frequency" from the local shelter's scanner, or that the US government considered forcing an incompatible frequency change. These claims were little challenged by manufacturers and distributors of ISO chips, although later evidence suggests the claims were disinformation. In fact, all chips operate at the scanner's frequency. Although ISO chips are optimized for 134.2 kHz, in practice they are readable at 125 kHz and the "125 kHz" chips are readable at 134.2 kHz. Confirmation comes from government filings which indicate that supposed "multi-frequency" scanners now commonly available are really single-frequency scanners operating at 125, 134.2 or 128 kHz.) In particular, the US HomeAgain scanner didn't change excitation frequency when ISO-read capability was added; it's still a single frequency, 125 kHz scanner.
That would basically mean that this particular kit would be optimized for these chips ... gosh... I guess my dog flap is not yet RFID-enabled :-D
Even better, it seems that the enforced norm in EU is ISO Standard 11784 or Annex A to ISO Standard 11785 (http://en.wikipedia.org/wiki/ISO_11784_%26_11785