Smoke is normal in an electric oven, but flames are definitely not.
In order to start a fire, you either need a spark, or you need to heat something beyond its autoignition temperature (AKA kindling point). You might have had a short - or you might actually be using a gas oven with spark ignition - but I'm guessing your issue was the latter.
Cooking oil or grease being heated beyond its autoignition point is one of the most common causes of kitchen fires (grease fires). Supposedly, some oils have autoignition points as low as 550° F (or 288° C), though I'm not sure which oils those are. Olive oil would be my guess as the lowest, but pepperoni grease could very well have ignited at self-cleaning temperatures (which, as you noticed, go up to nearly 1000° F).
Fortunately for you, all modern ovens have a mechanical interlock which prevents them from being opened during a self-cleaning cycle. If you'd opened it, you would have made the problem a lot worse by (a) supplying the fire with abundant oxygen, and (b) drawing all the hot air and flames out of the oven and into your kitchen, quite possibly setting your whole home on fire. Heat wants to move to where it's cold; that's why you keep your doors and windows closed in the winter.
There are a multitude of oven cleaners available for self-cleaning ovens - you are supposed to use these before you run a self-cleaning cycle. Yes, I know it's odd, but "self-cleaning" doesn't really actually mean that it cleans itself, it just gives you a little extra help. You need to try to clear out all the grease and big chunks of food first using one of these cleaners, then run the self-cleaning cycle to deal with anything you might have missed.
That depends on how large your oven is. I certainly couldn't do 6 whole fish in mine, but then I have a 24" wide oven. If you have a 36", it would be completely possible (these are standard US sizes; no idea what the equivalent would be in other countries).
Your main adjustment will be dealing with the extra mass to be heated by the additional fish, which will cause the oven to take some additional time to reheat. Again, this depends on how hot (BTU) your oven is; a hot oven would overcome the extra mass in a few minutes, but an underpowered (for this purpose, anyway) oven might take an additional 15 or more minutes, which would not be good for the texture of the fish.
So, It Depends. If you have a fairly powerful, large, high-end stove, I'd be ready to flip them at 10 minutes. Less powerful ovens will take longer.
Also, if the fish are individually closer together than 1", you'll have the issue that the edges of each fish will be soggy from the moisture of the fish next to it, and they won't get good texture. If they're touching even slightly, they will stick together. You'll also want to try to ensure some drainage in the pan, lest all of the liquid runoff collect under one fish and make it slimy.
it doesn’t happen all the time. Just off and on when using oven
Best Answer
Not really normal to see it happen frequently, though I have seen it happen in commercial convection ovens when they attempt to recover to a high heat. Or, simply, the oven was at 200 - 220 celsius, then the doors were opened letting out all the heat while pans were rotated, then the doors closed and the oven started to recover and heat up again while the fans were spinning up. You ended up with little 'poofs' of gas that ignited.
Throughout normal operation, however, it wasn't something you'd normally see. It worries me enough that my best advice is to discontinue use until you've been able to contact the manufacturer, or a repair place that's authorized by the manufacturer. You could have a faulty internal regulator which is something that can be pretty dangerous. It could also be a faulty part in your gas meter, which is a matter you'd need to discuss with your utility.
I'd turn it off, and get on the phone.