Hello - there's this joke/philosophy article on the 'net on how a prof shows the students how you can put stones in a jar - it looks full, then put some sand in it - it looks full and then you can still pour some beer in it.. It wouldn't work if you reversed the sequence. And I would also use this mindset for the decals - do the large ones first and the small ones later.
The thing with overlapping is an important point, but you have to research it a little and plan your work accordingly. Now if the insignia overlaps the stencilling - if you put the insignia first, you'll save some work. But on some aircraft the stencilling goes on top of the insignia - like the rescue arrow here:
Good luck with your build and have a nice day
Paweł