Pizzicato Professional is a music software that links all the features found in all other versions of Pizzicato. With the help of it, you can write sheet music, you can use all the intuitive composition tools and you can follow the full music theory course.
Bach-style 4 part harmony does follow some pretty rigid rules (no parallel 4ths, 5th or octaves, for example), and once you get your head around the rules it is a fairly trivial matter to harmonize a melody in four parts. That was the way I was taught at A-Level in the UK, and we weren't supposed to deviate from those rules - in fact, the deviation was pretty much an instant mark-down.
A really good website for learning about voice leading and chord changes (progressions), is
It is not a very instinctive site to navigate. But, much can be learned there to help with making good sounding changes via voice leading and chord selection. Be patient and be very curious, that is, poke around a lot on the site, & you'll find gems.