Fisty--I've skimmed this thread and I think you may have one big problem with your Rossi. If I understand suppressors, the idea is heavy and subsonic. If your Rossi is one of the newer ones it has a 1 in 30" rifling twist which is not conducive to stabilizing heavy bullets at slow speeds. My Rossi .357 will shoot std 158 gr bullets @ 1050 fps very accurately at 50 yds (1.5-2" groups). Those same groups open up to over a foot with many bullets tumbling at 100 yds. Push the same slugs up to 1400-1500 no problem but that doesn't help you. As long as you are talking close range you may get lucky.
Ps. Here is some load data that may help you. this is chrono data from a Rossi 20" .357.
Federal Mag case,mag tech sp primer, 5.1 grs of Unique, 158 gr RN lead=1071 fps. Es was 39 fps SD was 13 fps.
RP Mag Case--4.2 grs bullseye, mag tech sp primer, 158 RN lead=1069 fps
ES was 44 fps and sd was 13 fps.
If you back off a couple tenths on this load data you will be pretty uniform and subsonic. Hope this helps.