Well, if a "bracelet" or "ring" is "tiny", then it makes sense that some things could well be significantly smaller than that, especially if many of them can fit on those objects.
Otoh, if "any gem" is "tiny", then that includes diamonds and such that are much smaller than a link of chainmail.
I, too, have struggled with the desire to "houserule" small items, even to toy with the notion of a single pawn of vis enchanting several Level 1 or 2 items (tho' this was back in 4th ed). In the end, I felt that the rules as written were there for a reason, but it's definitely a judgement call.
Maybe the Hermetic premise never considered things as small as rats, or maybe it just needs to be a minor breakthrough, or, hell, just do it. It sounds like a Troupe decision.
I'll go with the Tiny applies to anything smaller as well.
Here's something interesting ... Individual Size (for animals and humans) is assumed to be Size 0. If you were to make the effect group, you might have a thousand really healthy rats (Assuming a rat is size -6. Group is 10 Size 0 individuals. Or 100 size -3 individuals, or 1000 Size - 6 individuals. A change of 3 in size is a tenfold change in volume.) running around.
So your really low level effect, for a group of size 0 critters, would be really effective for a large group of much small critters.