That's fairly normal activity for happy corys. Corys are very active fish, so they probably are just playing around.
As for your ammonia level, I wouldn't exactly say it was high, but no amount of ammonia is good for fish. The addition of some SeaChem Prime is a good to use for immediate relief, but this is should be saved as a short term emergency treatment.
From the activity of the corys I don't think that you are at this point yet. If they had ammonia poisoning they would be more likely to be lethargic and none of your fish seem that way. Their gills would also be a pinkish-red color like they were irritated.
Next you need to keep on top of your PWC. Be sure that you are vacuuming your gravel at the least once per week. Like most of the folks here have probably found out already, the best time to do a PWC is when you vacuum your substrate. You're already removing water to do that anyway.
You may need to back off on the food you are feeding your fish. You might be overfeeding them. Feed them twice a day only what the can consume in about 2 to 3 minutes. You should also add some sinking food for your corys a couple times a week.
As to your slightly acidic water, it's not that. Many corys come from blackwater conditions, so they like soft, acidic water.
