All but the most liquid symbols will be missing many minute bars throughout the day, let alone 15-second bars. If you want consistent bars you will need to create them yourself. My platform has a "plug holes" function for historical data. It adds missing bars before the symbol started trading (new symbol), after the symbol stops trading (no longer traded), and missing bars in the middle. You will find that after the 1,000 most liquid symbols, there will be missing days in a few symbols. This is not a mistake, they had no trades. Of course, during live trading, I build missing bars on the fly from the tick data and a 1sec message.Adding to the above post, the Dukascopy 15 sec data is also missing multiple 15 sec candles (within just one day), so won't be of any use.
The commercial trading platforms are toys and writing your own platform is a major job even for a very experienced programmer. If you can find something that does most of what you want, and you can program the rest, go for it. Howard Bandy had some books that he shared his AmiBroker and python code. Something like that may get you to where you want to be.