you shouldn't arch it too much, altough it usually isn't completly straight also,
you arch enough to get balance while incorperating the upper chest into it and pointing the feet upward, so that the angle of the whole body will be in line with the straps
like this:




planche is a little easier the inverted cross, also because it's harder to balance in it, i think they are both c
but maltese is harder, which is d, also azarian is d too, then there is victorian which is harder
they all work on different muscle, though, all the muscle in the body are contracted while doing it,
and all of the crosses work the arms and shoulders,
in addition
inverted works on the neck and upper chest , maltese works on the chest, victorian works on the back