[Lurch]

The idea of a themed army like that is great.

The new spider riders do come in a box of 10 with enough to make a full command in the unit. However they are not like normal cavalry models as the riders don't have separate legs. The rider is mostly a one piece model that sits on the flat part of the spider. It does however mean you won't need to do any conversion as the spiders are ridden by forest goblins anyway.

To make a troll look foresty try and glue lichen to its body to make it look like it is covered in moss type stuff. If you are good with green stuff you could even try and model a birds nest to go on its head.

You can actually buy forest goblin infantry units from the UK Games Workshop site as they used to be an actual unit. You can even buy the old spider riders made of metal, but the new ones look better. They are however outrageously expensive.