ORIGINAL: CanDo
John, you might get a kick out of this. Here's an extreme multi engine plane. It started as a Tower 40 trainer and my friend kept adding motors, one at a time, until he got up to 7 engines. I think the 3 engine version was his favorite. The motors were whatever he had and some loaners from friends, all used, and ranged from .25 to .40 size. Needless to say, he is a very experienced multi engine pilot and quite proficient at flying with an engine out. He came up with an engine pod with a throttle servo and a tank that allowed him to make any trainer a multi engine trainer. All motors were run a few minutes on the ground and run slightly rich.
I love it. Actually that isn't a bad idea at all, to make engine pods and rubber band them on the plane.
CanDo, many years ago I built a Godberg 1/2a Skylark and put on the two Cox peeWee .020 engines like they show on the plans. some days the engines would fire right up, other days it was a major hassle too. If a engine was stubborn, you had to be quick with the syringe to refill the fuel tanks before the first engine running ran out of fuel too. it actually flew Ok on one engine, albeit you weren't doing any stunts like that though.