I agree, 4-6 weeks is way too long to go without a water change. I do mine every week.
On the otherhand, I had both angelfish and neon tetras in the same aquarium, a 55 gallon. The angels are large now, over 4". I got them when they were little, body size around that of a quarter, and grew them up. I knew that they were very peacefull fish, but they were still over 3" when I introduced the neons and I didn't have any losses. Since then I have moved my angels to a 125 gallon. But not because of any problems with the other fish, its because I keep getting more angels (I'm up to 7 now, 4 adults 3 babies)