I'm trying to learn music theory, chord progressions, how to write melodies, etc.

[ImNotDedYet]

sounds good, got an example of romanticism pieces?

Beethoven was kind of a bridge from the Classical period to Romantic.

Some of my favorites from the Romantic period are Chopin, Mussorgsky and Rachmaninov, but there are a gazillion Romantic era composers if you do a search.

[768Sound]

The best way to learn about music is to find a teacher. That is to say, a person. Forget books, videos, websites.

I agree mate

[jancivil]

Do you keep the number of voices across your chord progressions equal, or change them up as you see fit?

I know that four-voice writing is a thing (big thing), but would you aim to only use three notes and then use four like in the case of the dom 7?

Writing for what? 4 part-writing is 4 part writing, you have 4 parts. It's a big thing as training for part-writing as, first of all it imposes that specific discipline. 4 part writing's model is 4 voices chorale.
The actual part-writing is going to be context-driven.
Look at some piano music and see what happens. Now look at a string quartet score.

Guitar I'd imagine would utilize all 6 or so strings when playing rhythm

Why? No. We are all free to 'change it up as we see fit'. Analyze some actual music.