It's the actor's choice whether to appear in your production. When joining SAG, actors have to sign an agreement and Rule 1 is "thou shalt not appear in non-union productions." So it's up to them whether they feel like violating that agreement or not. The producer technically bears no liability, you're allowed to hire whoever you want to hire.
Now, if you're a perpetual hirer of union personnel for non-union work, maybe someday in the future SAG would decide to boycott or picket your productions, or threaten their actors that they'll be kicked out of the union if they work on your productions or something like that, but the chances of that happening are pretty remote, unless the actor is a big-name like Jon Voight (who quit the union because they wouldn't let him work on a friend's non-union low-budget picture). SAG subsequently barred him from attending the SAG Actor's Awards show too. But they can't technically/legally penalize the production, they can only penalize/discipline their union member.
however when you try to distribute your product - you wont have paperwork clearance for all the elements (namely the actors) so many distributors might have an issue with that and just move on.
that is not completely true - while sag cannot come after you, if you get distribution the company will ask for clearance docs - the actor is SAG then they will want to see a sag singatory release possibly - be careful
I have directed several SAG features. Just call SAG and request the proper paper work. It is not that hard. And it depends on the state you shoot in as well. There are right to work states. But shorts are pretty easy to get around and would not sweat it too bad. Most SAG actors I know do non union shorts all the time and these are people with recoginizable names and faces.