Do I need an XML sitemap for just a few images and videos?

As others have stated, a sitemap isn't necessary for such a small amount of content. If you really want to have one, then including everything on one sitemap is okay.

Can you combine an XML Sitemap with an Image XML Sitemap?

You can use a separate sitemap to list images, or you can add image information to an existing sitemap.

Can you combine an XML Sitemap with a Video XML Sitemap?

You can create a separate sitemap just for video, or you can embed a video sitemap within an existing sitemap, whichever is more convenient for you.

Example

<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
        xmlns:image="http://www.google.com/schemas/sitemap-image/1.1"
        xmlns:video="http://www.google.com/schemas/sitemap-video/1.1">
  <url>
    <loc>http://www.example.com/videos/some_landing_page_with_video_and_images.html</loc>
    <image:image>
      <image:loc>http://www.example.com/image.jpg</image:loc>
    </image:image>
    <image:image>
      <image:loc>http://www.example.com/photo.jpg</image:loc>
    </image:image>
    <video:video>
      <video:thumbnail_loc>http://www.example.com/thumbs/123.jpg</video:thumbnail_loc>
      <video:title>Grilling steaks for summer</video:title>
      <video:description>Alkis shows you how to get perfectly done steaks every time</video:description>
      <video:content_loc>http://streamserver.example.com/video123.mp4</video:content_loc>
      <video:player_loc>http://www.example.com/videoplayer.php?video=123</video:player_loc>
    </video:video>
  </url>
</urlset> 

If your site uses a lot of graphics that are central part of your business strategy, I'd recommend you use one.

If it's just a few images / videos, perhaps skip the use of sitemaps as Google will take care of it.

Here's an excerpt from https://support.google.com/webmasters/answer/178636?hl=en

Follow Webmaster Guidelines and best practices for publishing images to increase the likelihood that your images can be found in Image Search results. Additionally, you can use Google image extensions for sitemaps to give Google more information about the images available on your pages. Image sitemap information helps Google discover images that we might not otherwise find (such as images your site reaches with JavaScript code), and allows you to indicate images on your site that you want Google to crawl and index.

Same goes for the videos as well but be sure to follow the guidelines mentioned here:

https://support.google.com/webmasters/answer/80471?hl=en

Creating an image sitemap is a good idea if your images load dynamically with JavaScript. It will make the easier job for web crawlers.

If you have that images available directly in HTML code, a sitemap for the images is not necessary.