Current junior anxiously thinking about recommenders for PhD applications next semester, any help is appreciated!

For context, I am interested in working in PDEs, more specifically fluid equations (Navier Stokes, Wave Turbulence etc....)

There are two teachers that I am confident would write me strong letters but I am unsure if the field they are working in would give me a leg up or otherwise in my applications.

1.) A general question I have is that do recommenders have to work in the same field that I intend to do a PhD in. I.e, if I want to work in number theory would it help to get a rec from let's say a Fields Medalist working in differential geometry?

2.) Of the two professors mentioned above one works in PDEs, but more so leaning towards the mathematical physics side. The other works in infinite graph theory, which is very removed from PDEs but I really want to ask him since we wrote two papers together. Would it help to ask them to write their recs more emphasizing my experience with PDEs?

Thank you. And wish everyone who is entering a PhD program luck, and best wishes for people who intend to apply for graduate school next year!