Not all sliders require images to be the same dimensions BUT if you don’t provide dimensions your photos will be cropped or scaled. Many sliders use the TimThumb function to scale images to fit a slider size. Layout is usually achieved by having a consistent dimension for the thumbnail.
You could find one that allows you to set a custom thumbnail size or you could try and specify a thumbnail image where by you use the page’s background colour to make the thumbnails appear irregularly sized. My guess is that you are going to have to compromise somewhere as most thumbnails are auto generated off of the original. Typically they have set dimensions. If your images have a set height then the widths will change, however typically the images are scaled to the longest side. If the image is wider than it is tall the whole thing will get scaled down leading to uneven thumbnail heights. The other option is to set the thumbnail to fill the space but that could lead to cropping on the thumbnail.
My advice would be to set it all manually if it matters that much to you. Create a set of dimensions you are happy with and make your images fit. Create some guides if you want a consistent height or width, because it is on the master template they will all remain uniform regardless of the slider or any scripts used to change the dimensions online.