Stability may or may not help. that is because it is a bottle of spores, it contains no live bacteria. The nitrifying bacteria and archaea which end up doing the nitritfying in tanks long term do not form spores, they reproduce by division. If you use Dr. Tim's One and Only or Tetra's Safe Start, the bottle contains live bacteria which are dormant. When you pour them into a tank, adding ammonia wakes them up and gets them working. The way you get ammonia in a tank is by adding it or by using fish (not recommended). Some people use fish food allowed to rot for this, but it is messy and inaccurate and there is no good reason for using it given the available alternatives.
What stability may do for some is to act as a bridge by using bacteria that may not be found in a tank once the proper bacteria have established. Also, a single addition of either of the two products above should be enough to jump start or even akmost instantly cycle a tank. Finally, most live plants arrive with the needed bacteria living on them/ So you get some jump start this way. Plus plants consume ammonium
What I am uncertain about is where you wrote, " I've been using Seachem Stability as well to help prevent any spikes" as well as what? How exactly did you cycle the tank the first three weeks, before you got the fish? If you actually cycled the tank, there is 0 reason to use Stability. Also, how often are you adding Prime and at the normal dose or higher?
Yes. SeaChem makes some excellent products, I use a number myself. However, Stability is not one of them, imo. SeaChem knows this but in the Aquarium game, everybody has to offer something in the bacterial starter product area, but only a very few actually offer what will speed a cycle and provide the bacteria that end up there anyway. SeaChem knows this- you can see that here
http://www.seachem.com/Library/SeaGrams/Biofiltration.pdf they do not make this easy to find either. (They also have the wrong bacteria for oxidizing nitrite in that article. The ones that will actually colonize tanks are Nitrospira.)
Your water params look fine to me for farmed Rummynose. I assume the numbers for KH and GH are ppm. If so you have hardness of about 6 dH.
General Hardness
0 - 4 dH, 0 - 70 ppm : very soft
4 - 8 dH, 70 - 140 ppm : soft
8 - 12 dH, 140 - 210 ppm : medium hard
12 - 18 dH, 210 - 320 ppm : fairly hard
18 - 30 dH, 320 - 530 ppm : hard
higher : liquid rock (Lake Malawi and Los Angeles, CA)
I keep Rummynose. I have a few with Altum angels at pH 6.0, 86F and about 60 ppm TDS. I also have a bigger group in a planted tank with pH 7.0, about 80F and 85 ppm TDS. They thrive in both tanks. (GH is a limited measure, conductivity and TDS meters measure much more things in the water than GH kits do). My water is soft and neutral pH out of the tap from our well.
I wish I could offer some thoughts on what is killing your fish. I am not sure what might be going on. Are the shrimp OK? Usually inverts are more sensitive to water conditions than fish. That might point to the fish having an issue when you purchased them but which only showed up when it did. Can you go back to the store and check out the Rummynose tank and see how they look? Look for any swimming weirdly, lethargic near the bottom or even dead. That would be a good clue. However, they may have sold out the batch from which yours came and restocked if much time has passed.