first of all - seems like stella is a lucky guy for finding his new home with you.... congrats to the new addition.
now to your questions (sorry, i have to be a little brief but i'm late for my dinner already
)
*) gcc are nippy - it's really hard to get rid of that kind of behavior... seems to be part of the lovely spunky-ness they come with... i usually pay attention to valo - by now i can read his body-language and when the signs indicate "there'll be a bite soon", i take him off of me and put him onto a play gym for a little time out. i'd try to pay attention to figure out how he displays that he's "pissy" (as i call it), and just avoid the close proximity that would allow a bite when he's in that minute... (valo usually only has that for a minute)
*) same's true for potty training - a lot of it falls in your lap... i'm not a big fan of total potty training for birds, but at least partial... valo has to go every 15-20min, and when he's about to, he backs up and lifts his butt feathers... hard to miss. i watch him and about every 15min i put him onto his poop-perch and tell him to, well - "poop". by now he goes... he still does his business when i forget to put him there as i don't really reinforce pooping on the perch, but it works well for us...
hope this helps