There are no union rates specific to short films...there's a standard day rate that applies to any production short or feature length. What most Independents, like yourself, do is offer their actors a day rate which is affordable to the production budget and then a percentage of the profits on subsequent sales. The down side for you as the producer is you have to give away a significant stake in your production and the financial books are open to scrutiny by all cast and probably crew, upside is you spread the risk!! Are we having fun yet?