Re-reading your post, I just had another thought.
We started on pureed carrots, butternut squash, sweet potato, swede.. (lots of sweet, orange veggies!) and all the purees did have water in them (from steaming the veggies) so they were quite runny (but not as runny as yoghurt, which I know is the consistency some books suggest). If the avocado puree seems to thick, then yes, you might want to water it down a bit and milk is obviously more nutritious than water, but I don't think it would matter if you added water instead.
I was writing my previous post thinking more about taste than consistency, iykwim.
Hope I'm not being too confusing now!
I guess, what I'm trying to say is just experiment with different consistencies and tastes and see what your LO likes best. HTH
PS I love your photo of her in the basket!