Ok let me give my comments to each bit of what you've said
The tank is a 55 with 2 rainbow sharks, 1 pleco, and 4 tiger barbs.
Do you know what pleco it is? There are lots of different types. However, they are all are poo machines and need a LOT of cleaning up after them so you have to keep this in mind when judging your maintenance.
I do water changes about 20% every week
I would be inclined to perhaps raise this to 30-35% each week and see what happens.
and change the filter pad about every month and a half.
Why are you changing the filter? Are you doing so because that's what the manual/fish shop says you should do? As ZZ asked above, what filter is it and what media is present? If there are no ammonia locking products in the filter then there is no reason to replace the filter pad AT ALL, EVER, unless it physically disintegrates in your hands. It'll get dirty, yes, but that's a good thing.
When I do water changes I add stress coat and nutrafin cycle to it.
Stress coat is good

Cycle... it is debatable whether or not it is has any benefit. Many members on here (myself included) have seen no benefit to such things at all, and so see them as a waste of money. However, there are those who suggest that they are useful for the actual cycling process. Regardless of which camp you are in, I am unconvinced of the need to continually add them once your tank is cycled.
No missing fish or invertebrates or anything. I am the only person with access.
Good to know. Sometimes dead fish or rotting plants can cause ammonia, as can other people being "helpful" by (over)feeding your fish for you.
I know that my pH has always been low for a tropical tank.
Has this "low" label been given to you by the shop, or did you work it out by yourself? What is the pH of your tap water? Is you tap water soft or hard? The reason I ask is that sometimes the staff don't know much about pH, and think that a nice round 7 is the only good number for pH to register at. Usually if your water sits lower than 7 it is because you have soft water, and if it sits higher than 7 it's because you have hard tap water. If you have soft tap water, and a naturally lower pH then it is good to get into the routine of doing larger water changes each week (rather than smaller ones) as this replenishes the minerals in your tank water. This is why I suggested larger water changes above.
But it is not low enough to try to correct it and my fish are all adjusted to it.
Good, you should continue to not mess with it. It is rarely necessary to try to change your pH.
Every once in awhile my ammonia or nitrate levels may be slightly elevated but usually when I do a water change they go back to normal.
That is worrying - it suggests that your filter is not fully handling the bioload that you have.