I have a 2003 calendar from my dentist printed on 3" x 6" 'refridgerator magnet.' Looks fine.
I'd say use non-serif font, like Ariel (or Helvetica). If you have Excel, do you have Word, too? Seems like Word would have calendar templates that would simplfy the calendar making considerably.
I would definitely create the calender outside of Photoshop in Word, Excel or whatever program; copy / paste it into
PS as a separate layer.
Sounds like you should be OK.
What is final resolution of image at highest .jpg compression? About 200-300 ppi ensures you will get crisp text.