I am planning to write a 15 minute play for a contest and I want to do some research but struggling to find any short plays so thought I could watch short films, would that help? Do people think a short film and a short play are basically the same; in terms of character development, plot etc
I'd say a big No, Blah they are not in any way at all similar other than the duration. For short films you (generally) write visually and for stage plays you should always write dramatically. They are possibly at the opposing ends of the script writing spectrum, so reading short film scripts would be totally counter productive.
I'd say read play scripts of any length, then condense them in structure. In a library you should find copies of one act plays. Read them but don't neglect the longer ones as many well known one act plays tend to be of the abstract type, and I'm not sure that's the type of play to write for a competition that wants to showcase modern dramatic writing. And remember that one act does not mean simply using the opening act from a three act play. It must be an entire play in itself.