Matt,
I popped in a 1.4 PC disc and it asked me if I wanted to format it. I'm guessing this is a good sign.
Not necessarily. An original (not FDHD) Macintosh SE was equipped with an 800K floppy drive, that would not work with PC disks. An SE/30 could format/read/write 800K/1.4 MB Mac disks (and 720K/1.44 MB PC, with the proper software installed).
Do you really mean SE/20? If so, I have to assume that it is a plain SE with a 20 MB hard drive. This does not have anything to do with the model name SE/30, which indicates a computer with a faster processor (68030, contrary to the 68000 in the SE variants).
You could test the (800K or 1.4 MB) floppy drive like this: Insert any PC-formatted HD diskette (the type with an HD mark and two square holes). Should the Mac recognise the floppy, with a "PC" floppy icon on the Desktop, you have a computer with a floppy drive for 1.4 MB (and a PC Exchange control panel installed). If the Mac instead asks whether you want to initialise the floppy, do so (at the highest possible capacity; Mac format is OK). When ready, double-click on the floppy icon and carefully check the information at the top of the open window. You should have something like 1.3 MB available if the drive is for 1.4 MB. If only approx. 785K (or slightly less), the machine has an 800K floppy drive, for 2DD/DSDD (not HD) diskettes.
There's a port in the back of the SE, but it's a pin type rather than Ethernet.
Are you referring to a coaxial connector (where an extension card would be installed)?
Jan