To fight the lvl 55+ and lvl 70 Fenrir battles, you DONT have to have SMN activated in any form or sort. For the lvl 20 ones tho, you do have to.

Once you recieve the avatar it stays with you, even if you dont have the job activated.

The ruby is RARE/EX and almost all important quest items are, and they drop randomly, Qufim and Buburimu Peninsula is what ppl say they drop most often.