your stoma is a loop and you still have the rod (bridge kind of thing) underneath the stoma to keep it from sliding back into the abdome. they use this rod for reinforcement to stablelize the stoma, then take it off after 10 days-2 weeks. this rod causes lots of leaks, so dont worry about it, cuz once they take it off, the whole appliance will sit much better around the stoma and leaks will be rare. this rod is also the reason for your itching. sure seals can really help, esp while you still have the rod.
leaks can also happen cuz of the stoma keeps shrinking and changing shape in the first few mons, so as summer said, you need to measure your stoma every time you change the appliance.
i use coloplast sensura 2 piece. like blueglass said, the opening is much easier to clean. also, i found their wafer stick much better on my skin. i also have a loop stoma and mine is really retracted, so i use a convex wafer (you cant use while still with the rod). i also use hollister's adapt rings #7805. but right now i don't recomment you the rings b/c of the rod. as long as you still with the rod, i suggest LOTS of paste around the stoma to seal all the gaps the rod is creating. i tried all pastes and liked convatec's paste best - it's the most solid and stable one of all the pastes i've tried. but once the rod is off, i highly recommend adapt rings of eakin seals.
the shoulder pain is normal. you probably had laparoscopic surgery, so they fill you up with lots of air. this air goes through the peritoneum omentum (it's a kind of an internal skin that covers the gutts). then it is transfered to the lungs and goes out from there. it takes about 2-3 days to be all over.
summer, how creative of that nurse to think of making those pin holes in the bag (really impressed). but hey, i'd say why work hard, better yet wear a srainer instead of a bag.
Sana, good luck, let us know how things go.