My past experience has been much the same. Started with UG filters (and they certainly have benefits, but also carry risks), moved to HOB, and now for the past 15 years canisters on larger tanks and sponge filters on smaller.
Canisters will (or should) restart with no trouble. I shut mine off (by turning off the power bar for each) when I do water changes, and the only time I have trouble re-starting is if I remove water so the intake is out of the water and air can get in. You can leave it running provided the intake remains below water, though I have noticed when doing this that when the water gets low in the tank the filter flow lessens too. I rarely do this, and wouldn't recommend it.
The issue with a power outage is the time; too long and the filter can be anaerobic. In very long outages, most recommend cleaning the filter before re-starting.
Back in the 1980's when I used HOB I came home one evening to the smell of burning metal. The power had gone off for a few seconds, and the HOB ran dry, burning the motor out.
I cannot hear my two Eheims running, and they have been continually running since 1996 and 1998. My Rena was noticeable at first, but now almost as quiet as the Eheims. Most air pumps make more noise than canisters should.
Only if there is some sort of water flow between the tanks themselves. The intake and outflow hoses have to be in the same body of water.
I know several members here seem to recommend multiple filters, but there is no benefit to this unless the filters are insufficient for the tank to begin with. Filters obviously take power to run, and while I have not checked into this, I would expect two filters (even though smaller) to take more power than one adequate filter. There is no benefit to having the tank water running through multiple filters, unless there is a biological issue to begin with. If a single filter is adequate for the tank's volume and fish load, it will do the best job of filtration, whether mechanical, biological or chemical, assuming it is operating properly. One also has to consider water flow (which should always be geared to the fish species), achieving this throughout the tank in a natural manner, etc.
Planted tanks have a bit different need when it comes to filters. The filter need not be as "obvious" with live plants because they do the job of filtration. Some water circulation and mechanical filtration is all you want in planted tanks. Again, this assumes the tank is biologically balanced; obviously over stocking the tank or having the wrong mix of fish species will have problems, plants or no plants.
This behaviour is most likely a direct result of being on its own. I'm not blaming you, I understand the issue...but unfortunately the fish does not. Shoaling fish that are denied an adequate number of their own species usually become stressed, and this manifests itself with abnormal behaviours, aggression or sometimes the opposite when the fish withdraws even to the point of not eating. The "security blanket" of sufficient fish is a significant need of shoaling fish that is a major part of their design. It is also a fact that adding more fish later may or may not remedy things. Stress when it is severe (to the fish) can cause detrimental internal changes in fish physiology.
Byron.
