I thought I remember someone telling me that the strength of astral creatures depends on who links the node, is this true? Because I think (not 100% sure) that the ones that come when I link are easier than the ones that tend to be there when I first arrive.

A quick answer? No.

The ones that respawn without linking are not random. The ones that come because of linking are random, and not based on the one linking. Aesyra's lobstrosities are much like mine. They range from mildly weaker to significantly stronger.

The confusion probably comes from a change that was made a while back to make astral mobs harder for L95 and above, but that change was reversed quickly afterwards.

Maybe they should make some of the spheres significantly stronger then the others. Come on, only fesixes and mitrans are a lil' weaker, others are almost the same, they only vary in afflictions - and even the worst ones give about the same exp as the less-horrible. I think.