To aid in the gelling of a lower sugar jelly, suggest you try the Pomona pectin. It will set without the use of any sugar. I would elect that even if you have a lot of added sugar, and the acidity of beach plums, you shouldn't be too concerned about the lower acid additions, unless your using up to or over 1/3 of each item in a single batch of jelly. With just a few for appearance purposes, there should be no safety issues.

When canning jellies with added low acid things, like garlic and jalapeno, it is necessary to add vinegar or bottled lemon juice. I can't say for certain how much would be needed, but you do need more than just the fruit juice to make it safe.
This isn't the site I was looking for, but it will give you an idea in the recipe for their golden pepper jelly.
If I can find the other one, I will post later.

Here is a link that might be useful: University of Wisconsin.