I do not know if this is just my computer, but please reupload the image with lower resolution
Placing meeple is never obligatory. You do not have to.
You can always choose which kind of meeple to put from your available pool.
Still garden can be occupied only by abbot.
Monastery by abbot or regular meeple.
Other features just by regular meeples.
so:
"Or can i chose wether to put a abbot on the garden/monastry OR a regular meeple on the field as a peasant or robber on the road."
yes, you are right.
I hope I made myself clear
Do not hesitate to ask further.