Great question Srijani Ghosh. You have heard that: "Nitrogen helps to make it acidic"; however, I would amend that statement too: "Nitrogen can help to make it acidic", as it will depend on a few factors. For instance, what type of N is the nitrogen in your fertilizer? It could be: urea, monoammonium phosphate, diammonium phosphate, ammonium nitrate.. so on and so forth. You might think to use an ammonium based fertilizer to acidify soil but by how much the pH decreases will be highly dependent on how much of the nitrate is consumed by the plants or how the nitrate is reacting with soil. So while it seems straight forward it almost never is. The same types of arguments can be made for phosphorus and surfer.
In general, I would not suggest using your fertilizer to adjust your soil pH. Specifically, when I am choosing fertilizer I am thinking about the following (in terms of the ratio's):
1) Am I establishing a new plant? - If the answer is yes then I am thinking I want more phosphorus or a ratio with a higher phosphorus content.
2) Do I believe my plants cellular structure is diminishing? - If the answer is yes then I am going to increase the ratio of potassium in my mix.
3) Is my plant lacking in color/do I want to "green" my plant? - If this is the case then I will increase the nitrogen content of my mix.
So, just to be clear, I am using fertilizer to target specific problems I am seeing with my plants and not to adjust for a particular pH. The problem, as I see it, with adjusting your pH with fertilizer is that you really have to know what's in your fertilizer or you might just end up burning or even killing your plant.
When adjusting your pH your first task should be to measure the soils current pH. Based on those measurements you can then go about fine tuning the soil pH. There are many methods of adjusting soil's pH which I would suggest you look into. If you notice deficiencies in your plants by eye or you have used a soil testing kit then you would adjust your fertilizer to compensate.
1) Gardenia- Yes you are correct they prefer a slightly more acidic soil. Specifically, they do well when the pH is around 5.5-6, 7 being neutral. I would use something like white vinegar to fine tune my soil's pH levels.
2) Sure, yes, you could use a 3.5/6/6 just be sure your soil needs phosphorus because too much could be a bad thing (soil testing kit). Another option would be to use bone meal.
3) No there is not. A 1/1/1 means equal parts N P and K whereas all of the fertilizer ratios you have described prior to this have been in unequal parts.
4) Chillies like potassium and too much nitrogen will reduce their fruit production. So, in general, if you see that their fruit production has decreased then you should decrease the nitrogen in your mix. You could start with your 3.5/6/6 if you have it on-hand.