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I'm a bit of a beginner, but I think scope 1 would stay at 150mm. The difference will be a focal length distance if the 150mm is at F5.5, and so is the 100mm (825mm vs 550mm). So your field will be different, smaller on scope one (given that you would be using the same sensor), but speed of exposure should be about/exactly the same.
Image circle here is, as far as I understand, dependent on the reducer and design on scope one, and the design of scope 2, you might be able to find that out by reading the specs of the scopes and reducer. Would you need to flatten scope 2 also? If so, the flattener will have a resulting imaging circle in its specs.
I use stellarium to tackle these sorts of questions, you can input your focal length, reducer magnification, sensor size, and then switch the view between them, see how different objects frame up. This is how I've chosen all of my scopes so far, which is probably sub-optimal, but I've not been able to look through many due to covid-19
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