I always hated Finale’s Rhythmic Notation slashes! The smaller slashes didn’t pair well with the regular slashes and the ridiculously long stems were goofy.
I have “oversized” selected and it should be displaying my whole/half notes as diamonds but as you can see, they’re still those weird rhombus-looking things
That setting affects Slash Regions and Slash Voices but …
… you aren’t using those so it will have no effect. If you are manually changing the notehead set, it will use whatever set you select. You probably want to be using a Slash Voice here, but if you don’t want to for some reason, you’ll need to choose a different Notehead Set like Slashes / Oversized Slash Noteheads.
My charts are usually mixed with default noteheads on notes/measures where the musician needs to play a specific line and also rhythmic notation to denote specific band hits.
Yes, so it will use whatever noteheads you have defined in your Slash Noteheads Notehead Set. That has nothing to do with the Engraving Option setting which specifies how Slash Voices should appear.
Below I’m using a regular voice, a Slash Voice, and a Slash Region (in green):
You do have to use Remove Rests when combining voices. I can’t remember if there’s a default shortcut for that or if I defined it, but you’ll definitely want to use a shortcut if you do this a lot.