Illusory spores sound fun.
I'd have them associate with snails but leave it as flavor text.
The size business is a bit weird. The table in the MM glossary says Fine is "6 in or less" in length, while Diminutive is longer/taller. However, the table in the PHB lists the example critters as being a fly for Fine and a toad for Diminutive. Now, 6 inches is a pretty darn big toad. Which takes precedence?
All fey get low-light vision (unless otherwise specified) by the rules for the type. I wouldn't make them a sprite; for some reason they seem a bit less magical from that picture. But I'm just spitballing. Maybe Nookie has some preference?
That's a fair point. But, if we give it SLAs, I kind of feel like giving it more plant (and maybe animal) centric ones. Like druid spells.