I do upgrade fire departments to only use helipads. They can just fly right across the map and put out the fire. Get there faster & don't deal with traffic. They're actually less efficient if you leave fire trucks in the game, because they prioritize the trucks, which will dispatch first and block the helicopters.
The sewage problem is probably due to time of day - sewage needs seem to increase around the transition times (6 am & 6 pm). I would slow down the game and watch the sewage map to see when you get the message, then transfer to the other city and wait for the synch to get around the same time. You may find the usage numbers lean towards needing more capacity.
You can also get caught with sewage backups if one city grows significantly before you play the other city. There will be a synchronous time period, where you just have to wait it out before it registers that you added more capacity.
Another tip - build your sewage system right next to your water supply. The sewage plant will filter the water and dump it into the water table, creating unlimited water supply, no expensive filter pumps needed.
Happy building!