Ok, I've been doing more research and it looks like I should be able to can yellow heirlooms. I'm still a bit concerned because they are a sweeter variety though. Should I add additional acid - or stick to the basic Blue Ball canned tomato / Annie's Salsa recipe (my two preferred recipes for canned tomatoes) and assume they've accounted for variable acidity of garden tomatoes?

A Ball recipe or Annie's salsa will work just fine with any tomato variety. They're designed with sufficient acidity to cover lower-acid varieties or just natural variations in acidity due to ripeness/growing conditions. (I am assuming healthy tomatoes from healthy plants.)

There are some few tomato varieties that are lower-acid. Jetstar comes to mind. But it has nothing to do with color or whether they're heirlooms or not. Most of the tomatoes people THINK are low-acid just have more sugar so they're sweeter. A yellow tomato is no more likely to be low-acid than a red one.

Carol

If you're talking about the small-ish fuzzy yellow ones (there are several tomatoes called "peach") I've used mine to make a wonderfully unique tomato juice. The sweetness also lends the juice to making wonderful smoothies, which are also very healthy!

Carla in Sac

Never guess at the acidity, no matter what kind of tomatoes you grow. Tomatoes can be low in acid or high in acid and it can also depend on the soil conditions, weather, and any fertilizers used. Its always good practice to add the necessary acid to each jar. Citric acid or bottled lime, or lemon are necessary.

This recipe for salsa would be perfect for them !
My friend made this recipe and it is good. She made more after the first batch they liked it so well.

Here is a link that might be useful: Peach, apple and tomato salsa.