Fire-bellied toads are invariably wild caught and will have parasites of one kind or another. I would tear out the entire contents of the tank and dispose of it, and then use a 5% bleach solution on the tank itself before starting over again. I don't see a way you can clean a GS background.
The thumbnails you mentioned _are_ tiny. They are also not bold. There really is no such thing as a bold thumbnail, just one that is less shy than the others, and frustratingly, this can vary from individual to individual. Out of the ones you named, the Tarapoto have been the most bold in my experience. I certainly wouldn't keep benedicta or lamasi as my first thumb.
Thumbs are not that hard but there is a learning curve to darts in general - once you get a feel for it, darts are among the easiest frogs to keep, if not the easiest of all.
I can sell you a sexed pair of Tarapotos, and slightly less reliably sexed pair of Varaderos. However, I think you should start with a tinctorius of one form or another - they are far more bold, they tame well and will beg for food from you, and adults are almost constantly out and about doing something right in full view. You can get sexed pairs (I just bought a sexed pair of Matecho) but they are usually pricey (at least $225, more if proven). I would suggest going through the tinctorius morph guide and seeing if anything grabs your eye. The photos there are not the best but you can plug the morph names into a google search.