In addition to what MitchM said there's at least one more interaction to note and that's algae. It doesn't really matter, but just for fun, algae will consume ammonia directly bypassing the nitrite/nitrogen deal. A pond doesn't need to be visibly green, there can be a lot of algae before visible. Cycling a water garden is almost never needed, the light fish load in these is handled fine. It's something done in high fish load systems. Bottom line is testing ammonia at the start tells you what's going on. It doesn't really matter in a water garden whether it's algae or bacteria consuming ammonia, but in virtually all water gardens algae is doing the heavy lifting.
Rain water normally is 0 KH so the water isn't buffered at all. That leads to PH being very unstable and readings being kind of meaningless. If you measure KH and add (if needed) to keep KH up you don't even have to test PH. Rain water coming down is generally very low, like in the low 5's in west US and under down to like 4.3 in the NE US. In thunderstorms it can get down to low 2's. This can have an effect in low/no buffered ponds. And of course the resulting PH from increasing KH can effect the ratio of toxic and non-toxic ammonia. Also, chemicals to dechlorinate water can result in meaningless ammonia test readings for most common test kits.
Bacteria in a bottle is very popular with stores and people. The better the company marketing the happier the people. It's a great placebo and helps many people lower their stress level. However, if you're part of the shrinking world of people interested in data you will find no hard evidence bacteria in a bottle has any effect other than add a bit of waste to the water. If you prefer a common sense approach you can add some ammonia to water, in a bucket or whatever, and be amazed how fast bacteria set up shop and consume it all. Ammonia converting bacteria is everywhere. As for exotic bacteria some companies elude to, well, back to placebo. For sure many people who pay $20-60 for a bottle of stuff do claim to "see" benefits. Most of these products seem to do no harm so if you feel the need, I'd say do ahead. It's just money.
If you have the time, for most water gardens, it's hard to beat filling your pond with water and just waiting a week or more to add fish. You get stable water pretty fast. But a testing ammonia and KH is good insurance.
