Do you mind using the lightbox style or are you wanting to display it in a page? Really the only limitation is how much space you have to play with and how you want to present the gallery/slideshow.
If you use a lightbox slideshow then it is easy to have both thumbnails and images display on screen and size it so that it will show each image as big as possible. Alternatively if it is bigger than the screen will allow it will scale itself down. This is the easiest way to get all photos to display with a mix of odd sizes or shapes. The computer is sizing them to fit the screen.
If you want to insert a gallery into a page, chances are that you are already limiting some part of the images. If in your case you are limiting the height, you will likely find that all your images are pretty close horizontally. One thing to note is that although the sliders ask for dimensions, the images themselves DON'T have to be these exact dimensions. So for arguments sake you have 10 images and they all have different heights and widths. When you insert your Slider it asks for dimensions. That is the maximum dimension any image will be able to fill. You can choose whether each image will scale up or down or stretch to fill the display space. This means you don't have to worry if your image is smaller or larger than the area, the slider will make sure it fits.
This means in practice you don't have to have images that are all uniform on either height or width.
So it is a matter of presentation style. Do you want to show the photos in the context of a page, or dim the lights and let your users screen be totally covered in images.
All this to say you have lots of choice, it will just be implemented slightly differently.