I think the Eureka Treeman would be too large for a Dryad base. Keep in mind that the images of Orion, Treemen and Warhawk riders above are all on 40mm round bases, not 25mm (the Stag and foot heroes are on 25mm bases).
The scale issue is also significant. Attached is a quick photo of four different models from different "Wood Elf Ranges". From the left: Eureka Wood Elf, Eureka War Sprite, Copplestone City Archer, Pendraken Wood Elf
As you can see, the Pendraken elf is almost twice the height of the Eureka one. For some reason the Eureka war sprite is also much larger than the wood elf from Eureka. I don't think any of these would mix well, expect maybe the war sprite with the copplestone models (though that's the least useful combination). Regular GW models are about the same size as the Eureka models (in between the Wood Elf and War sprite).
Measuring from toe (not base) to top of the head/helmet, I get:
Eureka WE:10mm
Eureka WS: 12mm
Copplestone: 14mm
Pendraken: 15mm +2mm thick base (though this model has a bit of a stretching pose)
Hope this helps.