First of all, you don't need to use any different powders in an 18 inch barrel than you would in a handgun length barrel. The powder, whether slow or fast, is used up in the first few inches of the barrel anyway. It's a myth that you have to use fast powder in a short barrel and vice versa though it seems logical intuitively.
Secondly, don't use rifle primers in 45 ACP cases. You don't need them with any powder you might try in such a small case. They probably won't quite fit flush with the case head since pistol primer cups aren't quite as thick as rifle primer cups and will cause premature high pressure in such a small case. They will also worsen accuracy in all likelihood.
Welcome to the forum, by the way.