Photoshop :: Scanning Dpi Size For Vintage (3x4pic) To Produce Larger 11x14 (print Output)

I am creating colage images in Photoshop CS4. I sometimes use scanned vintage daguerreotypes or other small snapshots. They are often small sized at 3.5" x 4.5" or 4.25" x 5.5". My Epson 835 scanner can scan in at 300, 400, up to 800 dpi. If I intend to print these scans at 8x10, 11x14 or a larger size within the overall collage, should I scan in at the highest dpi allowed by my scanner? Does this make a real difference after 300 in resolution/clarity?
 
Also, Does Photoshop CS4, in the enlarging process, as I bring the smaller image into my composite and stretch size it up with (T) tool, end up just making "junk pixels" using "Bicubic" or "Bicubic smoother" in the printing from flattened PS  file, even tho I scanned in at a high dpi? scan in at best dpi so that the image won't be pixelated, lose resolution as it is used larger inside the final composite collage.