A couple of people have asked about scanning "odd sized negatives," and I don't see any response in this thread.
All the "negative carriers" provided (or available) for most scanners do is line up the negatives in little slots or pockets to make it easy to get a straight scan. If your platten and the backing pad are really clean you should get the same results just by laying the negatives out on the platten. Since you'll nearly always want quite quite a lot of enlargement when you print from small format negatives, the higher resolutions at which you'll want to scan can make tiny bits of crud look like boulders in the print.
Note that the emulsion where the image is recorded is on one side of the film, and some scanners have so little depth of field that it can make a slight difference which side up you lay them down (and you can end up with lots of left-handed pictures), but most scanners won't really care about which side up they are so far as the focus is concerned. Rarely, you might find a scanner that does better if you "scan left handed" and flip them in your image editor when you clean them up to save; but unless you're really fussy you probably won't be able to see the difference.
Since "negatives" have inverted colors, turning them into "positives" can be tricky. Some scanners may have "automatic" color inversion programmed in, or you can "invert color" in some general photo programs, but the "red, green and blue" that the program works in may not use exactly the same red green and blue that the film used. Additional color conversion traps appear if the program uses CMYK color space instead of RGB.
Additionally, the colors recorded in the film are intended to produce the correct colors when light is transmitted through the film, and the light reflected off the film, as in most scanners, may be different. This difference between transmission color and reflected color may also be visible with color slides. True "negative scanners" should use transmitted light; but usually you can work with the refelcted light scans made by the common kind.
If the negatives (or slides) are more than a year old, the fading inherent in most film emulsions probably wipes out any visible effects from the above details though, so try to be happy if you get something that looks good to you.