Depends on look, thinner threads blend, thicker threads show off the quilting you can always do a mixture as well depending on look attempting to achieve

I have stuck with one thread (Aurifil 50 wt), both top and bobbin, and am ready to expand my horizons! Can different weights of thread be used for the top and bobbin? I'm pretty sure the answer is yes - but are there certain guidelines I should be aware of when experimenting?

Also, while I am not showcasing my quilting at this point (I tend to either stipple or other simple meandering stitches), I don't want it to be invisible either. I would like to try polyester (perhaps something like So Fine). I tend to like solid colors, so that rules out So Fine #40 from what I see. Would So Fine #50 disappear into my quilt?

Yes you can use different weights ... you will need to adjust the tension.
Make a sandwich of the same fabrics and batting, to practice til you get the tension figured out.
Plus a good chance to get used to whatever quilting design you want to use.

Threads ... generally, the closer the colour match the more invisible your quilting can become,
and let the oopsies disappear!! Likewise the finer the thread.
I have done miles of FMQing using 100 wt Invisafil.

The coarsest I have used is 35wt.

only big changes I'm aware of is that some new, strong but very fine poly threads have been added, like the Superior Microthread. I'd also add that Superior thread (maybe other companies too but I usually choose Superior) has printable charts for thread, needle usage. Even if you use a different manufacturer, you can match to the Superior chart to get correct needle size.