ArM5, page 102 says that lab texts are only of use when recreating the same enchantment. There is a little freedom in that the new device need not have all the enchantments of the original but there's one line that could be seen as ambiguous.
"in the case of an enchanted item, the item itself must have the same shape and material as the one described in the text".
Personally, I read that "shape and material" as only refering to the shape and material table and that the item design may differ. For instance, I enchant a great two handed iron sword with an effect that slays a man with a single blow (as though a sword like that would need it...). My understanding is that I could then use that lab text to enchant a somewhat smaller short sword with the same effect. After all, the new sword is iron and is a sword. That the design is very different doesn't make much difference in magical terms.
Are there any reasons why the above doesn't work?
And what about if the larger sword also had a ruby in the grip, perhaps used for another enchantment? I would still take it that the lab text could be used for the smaller sword as the key shape and material bonuses are present.
And what about the other way around? What if my great sword is pretty simple and my short sword had all the extra material bonuses? The same iron + sword is present so I'd allow the lab text to be used.
Am I being a little lax with the intent of the rules or do these seem fair enough?
I'm a bit torn - for a greater invested device, I'd want to make sure that all the magically relevant elements are presents, including the way the device was opened, at which point size and components both matter. So, whether or not a given bonus is used in the effect, or even if it gives any bonus at all, I'd be loath to have them changed. On the other hand, when bits break off, the entire device isn't necessarily ruined, so you should be able to get away with variations, especially immaterial ones. For lesser invested devices, I think it would be good to be strict: you got an all-in-one recipe from which you cannot really deviate and hope it'll still work.
Anyway, your approach is fine if you want to run it that way; I don't think it would break the game, though variating from the lab text should at least warrant a roll on the experimentation table. And I don't think either that you'd be able to stick the spell in a dagger with that lab text, even if the spell would otherwise fit in such a smaller weapon.
As per the RAW I would say that you need the ruby, but that the sword can be a shortsword or a greatsword or whatever: it is a sword.
Still, I would prefer the lab texts not to be flexible myself. But that is me. Whatever rocks your boat is fine here it is no biggie anyway, but the moment that you can use the same text to enchant a slingstone and a boulder....
If it's a greater enchantment, the "size" must be the same, but it sounds like it is. (Even if it's a lesser enchantment, perhaps the "size" would also dictate if the two forms are like enough? At some point a sword becomes a dagger, just as a boulder becomes a sling stone, and those are both clearly diff.)
The problem is that a lesser enchantment cannot be additionally enchanted - but it's up to your troupe whether that ruby counts as part of the sword or not. In theory, if that's kosher, than what appears to be a single object could, in fact, be a compound object made of infinite numbers of lesser (or greater) enchantments, all bound together by mundane means, layer upon layer over decades or centuries of additions, each one "separately enchanted".