a) Tearing out the center - A lot of similar recipes actually tear out the center, instead of squishing it to the sides. Whilst this makes more room for the filling, it is an obvious waste of bagel. Squishing to the sides does make a more dense and doughy texture, which I actually really like. If you'd prefer a thin bagel, the above filling will probably only fill 3 instead of 4 bagels (depending on how much you tear out).
b) Different Fillings - Feel free to switch up the fillings! However, I definitely recommend keeping the eggs/cream/cheese for consistency. Also If you're subbing a considerable amount more veg, I'd fry off some to get rid of some of the moisture, otherwise, the quiche will be slightly watery.
For the perfectly sliced bagel get Eric's Bagel Slicer