When the rules says 8R or S8R, it is normally for ease of writing. In most cases, it should be understood as "the longest side (vertical or horizontal) must be 10" (8R) or 12" (S8R)". As crop***g is part of a photographic presentation, it should be accepted in competition. During judging, some judges would make comments like "this picture will look better if the author crop off the top/bottom" or something liek that.
Film formatt is not a problem so long as you can get the lab to print it at 8R or S8R, unless it is stated in the rules that must be taken on a 35mm/135 camera.