Before firing your 1919, you need to check the head-space. Firing a 1919 that is not properly head-spaced can damaged the gun and also be of extreme danger to the shooter and bystanders. Once you head-space the 1919, it should not require to be head-spaced again unless you do something that requires screwing the barrel in or out (disassembly or changing barrels). However, it's probably a good idea to check the head-space every several thousand rounds just to make sure it hasn't changed due to parts wear.
The 1919 is not designed to be head-spaced around a certain manufacturer of a cartridge. You head-space the gun using the below OFFICIAL method, which does not require a cartridge to perform the head-spacing. After you perform the head-spacing, the gun should work and operate safely with any ammo that is in-spec.
As long as you are using high quality in-spec ammo and a in-spec barrel (aftermarket barrels may be of poor quality and out-of-spec), and following the official head-spacing method, you need not be concerned with whether you mix ammo or not. Follow the below OFFICIAL head-spacing method, use in-spec ammo and a in-spec barrel, and you will be fine. You should not be trying to adjust head-space to accommodate for a bad quality out of spec ammo, and/or a bad quality out of spec barrel. If your barrel or ammo is bad quality and out of spec, you are risking injury (possibly serious) by trying to adjust head-space outside the official procedure to accommodate that out of spec item.
http://members.roadfly.com/agent7/guns/headspace1.jpg (head-space page 1)
http://members.roadfly.com/agent7/guns/headspace2.jpg (head-space page 2)