You can probably get away with stock settings for the most part.
You’ll want to experiment with the line multiplication settings for 240p and 480i to find out what looks best for you and what your monitor will tolerate. For example, 240p/288p line2x should work fine, but the higher the line multiplication you can go (especially if you can get to 5x@1600×1200, the sharper the resulting image will be.
I expect 480i will be a bit of a pain, because few PC monitors are known to deinterlace; so your monitor may outright reject 480i passthru and line3x. Line2x and 4x for 480i, resulting in 480p and 960p respectively, should work; but those modes force on bob deinterlacing, which looks horrid. If your monitor will deinterlace and take line3x 480i, it’ll look great.
As for toning down the jaggies, I think the only options you’re going to have are 1) use the lowest line multiplier that is compatible with your display, or passthrough, at let the monitor do the scaling; or 2) turn on hybrid scanlines, adjust to your desired thickness, and then turn up your display’s brightness to counteract the dimming effect of the scanlines.
Just keep in mind that, if you are going to use scanlines, I think it’s ideal to ensure that the target output resolutions of the OSSC are integer scales of the display’s native resolution, otherwise the scanlines will be unevenly scaled, and you’ll get alternating thin and thick scanlines–it’ll look bad.