You mentioned Little Dude is a possible failure-to-thrive bird. His small size could indicate organs that are also inadequately sized to operate efficiently. This includes his gizzard and liver. A gizzard not up to the job may be too slow to empty the crop. A liver too small may not be coping well cleaning his blood of toxins so they may be building up, thus the yellow urates. He could also have an infection causing these yellow poops.
Where to start treating? Doing a Corid round certainly won't hurt. An antibiotic, if you have one on hand won't hurt. If he has an impacted crop, oil won't hurt. You can safely treat for all of the above as well as worming all at the same time. I think he's got enough symptoms where some radical action is warranted.