Regarding the second question, I'm a history major at UCLA, and ever since last year, when I learned a bit about the Spanish Civil War era in a class for my Spanish minor, I've been thinking of going to grad school to research that topic. However, our department is light on professors of Spanish history, and I don't think we have any in that particular field (so I haven't taken any classes on 20th century Spain). A professor I spoke with recently (who studies a different area of history) advised me to apply to grad school in a field I have the most experience with - which right now, seems to be early modern Spain and Latin America, counting the classes I've taken for my major (and will take) as well as certain literature courses for my minor. Is this good advice? Do you professional historians on this subreddit have anything to add?
I have no direct experience with US grad school, so am loathe to give any advice about how to best work within that system. That said, the Spanish Civil War is certainly an active field at many institutions over there, and it doesn't seem unreasonable that you could find a decent topic to work on. The main difficulty you'd face is that the historiography can be a bit difficult to navigate, as personal and political factors still drive a lot of what gets written (and, importantly, how it gets written). Without a lot of reading and a bit of guidance from a prospective supervisor, framing a convincing research project won't be easy.
My advice would be to seek out historians active in research and supervision whose work you are interested in, and get in touch directly. They're ultimately the first person you'll need to convince that you have the skills and knowledge to work on the topic area, and if they respond constructively and helpfully, then that's a good sign they're worth working under.