I've experimented a little in this with different fruits. First off, I've found that using a juicing machine typically results in an unappealing texture for a soda, but allows for some interesting fresh fruit combinations. As good as modern juicers are, you get pulp in the juice and even with good filtration, some pulp may settle out in the soda. Larger batches can prove difficult to filter with crude home-based filters like coffee filters.
I tried simmering and boiling and wasn't too successful at getting a great extract with most fruits I tried. I have meant to try doing something like shredding in a food processor first, which would certainly improve extraction results, but haven't gotten around to it. I haven't tried a steam juicer, so I couldn't tell you the results of that method, nor provide advice.
If you want to play with a batch, I recommend scaling recipes down to about 4 or 8 ounces to taste different ingredient combinations. I would prepare samples of my desired ingredients, combine in their respective percentages and taste test my mini-batches until the desired flavor was reached. Obviously these aren't carbonated, but give you an idea of the final taste and texture. This also allows you to dial in adjunct flavors, such as lime, that can easily overpower.
Also, you typically want to make sure you combine with water, because straight fruit juice can be pretty sweet. Diluting also helped reduce the time to bombing in the bottles, since I yeast carbonate. I typically would consume a gallon in a couple weeks, so I haven't done long term storage and suspect it's vulnerable even with excellent sanitation techniques.
I would typically bring the mixture up to around 165 and maintain it for around 10-15 minutes. Milk pasteurization techniques would also probably work, which is 145 for 30 minutes.
That's my experience...hope it helps!