reduce height of score

Don't do anything until you have entered all (or most of) the notes. Then hide the empty staves:

Format >Style >

I understand that you on some pages have one system only (consisting of 6 staves) and on other, MuseScore manages to fit two systems (with 2 x 6 staves).

The reason this happens is that not all systems get the same height. Some are higher and some are lower depending on what notes, lyrics, staff text, rehearsal marks etc.

You have two basic approaches if you want two systems on all pages.

1. Try to make the systems that are too high, lower. Sometimes things are stacked on top of each other like a rehearsal mark over a treble clef or a volta line etc. moving things around a little bit might be enough.
2. Reduce the scaling, making everything a little bit smaller.