I have never used that polish, but it seems like it is a fine polish and it seems like 105 would be a little coarser if ultimate doesn't work well enough. I could be mistaken. As always, use the least aggressive methods first. I see no reason why you could not use ultimate polish with a DA. Assess the severity of the scratches and proceed accordingly.
Each situation is different, but realize that SS paint is much softer than clear coat, and so be wary not to expect to polish it as much as you would clearcoat to get that severity of scratches out as you would out of clearcoat.
( I incidentally, have the opposite problem. Most of my polishing experience came from polishing SS paint, and I am constantly frustrated as to how difficult it is to polish basecoat/ clearcoat.

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If they are about the same throughout, then do a test spot with the ultimate and see how it goes. If there are spots that are particularly bad and some that are really light, realize it would not be prudent to use the same degree of polishing on the whole area in order to preserve paint thickness. ( you would not want to polish the light scratches as much as the deep ones just because it is what it took to get the area where the deep scratches gone completely.)