In theory it's as simple as:
var inimigo = load("res://inimigo.tscn").instance()
inimigo.position = Vector2(1337, 42)
inimigo.scale = Vector2(1, 2)
If you get an error about a "null instance", then you're trying to access a node that simply doesn't exist at that point in time! As you have neither posted your full code nor the line number where the error occurs, I cannot help you with that though.