SetLand says "put it here"
Land says "wipe it clean then put it here"
So if you are setting the basic poptype it looks like your only option is to wipe the province clean then set both poptype and defenders.
Here is part of
http://www.dom3minions.com/docs/map_qref.txt
6.1 #land <province nbr> kill off province and allow settings
6.2 #setland <province nbr> do not kill province but do allow settings
6.3 #commander "<commander type>" place commander and allow settings
6.4 #comname "<name>" overwrites the active commander's random name
6.5 #bodyguards <nbr of guards>"<type>" Gives bodyguards to commander.
6.6 #units <nbr of units> "<type>" squad of soldiers to commander.
6.7 #clearmagic Removes all magic skills from the active commander.
6.8 #xp <0-900> Gives experience points to the active commander.
6.9 #mag_astral <level> Gives magic ability to the active commander.
6.10 #randomequip <0-4> Gives random magic items to commander.
6.11 #additem "<item name>" Gives a magic item to active commander.
for type I find that the number assigned to a unit works better than the name. So to finish out your amazon example it would be...
#land 33
#nostart 33
#poptype 40
#commander 347
#bodyguards 10 248
#units 20 367