People get frickin wound up about resolution, but honestly the resolutions that your eye can detect are FAR below the resolutions that your ordinary home scanner can produce. Unless you're planing on printing these the size of a building or viewing them on 25K screen in 2098, don't worry. Scan on a photo setting, in a sympathetic format that won't result in any data loss (maybe tiff), and get on with your life.
The last time I scanned a picture on the scanner I owned at the time, I could definitely tell a difference. The original was a digital picture that had been printed out as a holiday card by shutterbug or something, so maybe it was not as clear and sharp as my eye perceived it to be? Because the scan was unacceptably grainy, especially in comparison to the original.
Anyway, your post is reassurring in a "you almost can't screw this up" way.
Yeah, scanning a cheap print is not going to give you a good result (unless you knew how to manipulate that. A professional can and would scan a cheap print VERY large, for printing rather small, so your eye would not see the print dots). Scanning a photo print, be it digital or darkroom, will be just fine. You won't be able to tell the difference. Scanning a slide will be just fine.
Regarding resolution/file size: I think the take-away from this thread is that bigger is not better. The technology has eclipsed the differences that your eye can see, which makes it completely pointless, imo. The home scanner of today is better than a professional drum scanner of even 15 years ago.
If it were me, I would approach the situation like this:
1. Divide your pics into those that need correction, and those that won't. For example, many old photos get a yellow cast that's easily corrected. If you can use Photoshop, you can correct this, but these may be the photos that would be easiest for you to farm out.
2. Set up your scanner for 600 dpi, tiff format
3. Drop groups of shots on the scanner, and record what they are on a word doc eg Little Billy's 6th birthday
4. Name file appropriately eg Billys_6th_Group1
5. With the photos still on the flatbed of the scanner, change scanner to 600dpi jpg, scan again with same file name.
6. Repeat until you're finished
7. Go into file and grab all the jpgs into a separate folder
8. Save your index type document with both the tiff and jpg groups.
9. Stick the files on pen drives or discs, or whatever you've chosen.