... but this is for nitro not BP so as I understand it Alox should be fine.
The advice you’ve had above is sound. I’m not sure if Herco is hard to obtain as it was a few years ago. If it is then Alan Westlake recommended Blue Dot as an alternative.
I would call him and ask his advice to start with if you haven’t already.
The one thing I would caution is that I have heard a number of shooters say ‘well I know the load I used to use in my .357 cartridge revolver, so I’ll just use the same in my .357 nitro revolver’
I checked with Westlake on this one and the way he explained it is that because you are pushing a projectile that has been pushed (relatively tightly) into a steel (relatively hard) cylinder, you need a slower push to make it leave that cylinder than you would need to make a bullet leave a brass (relatively soft) cartridge case. Therefore, using loads of relatively fast pistol powders can make for dangerously high pressures.
It's all forgotten now, but Rockers and Mods only started fighting as the Rockers were annoyed at having some of their comments removed