I would suggest that Al Kindi and Plotinus give the 'non-dual' description of unity and multiplicity. These opposites must be reduced for a fundamental view. The language here is tricky and can be misleading. The opposite of multiplicity is not unity but singularity, while 'unity' would encompass singleness and multiplicity within itself. The word would not imply a numerical value. Thus to say the Universe is a Unity is to say it is both Many and One. Unity allows us to overcome this logical antinomy and all others.
The idea that Unity cannot be described or conceptualised is necessary to this view as is the inappropriateness of predication to the True One. Bradley discusses this in Appearance and Reality where he writes that predication (thus language) is necessary for metaphysics but also illegitimate and misleading. As Lao Tsu puts it, the 'Tao that is eternal' cannot be spoken.
Causation here should not be thought of in our usual way. For this view we would need to replace the idea of causation-as-action with that of identity. The world would be the way it is not because it is intentionally caused but as an ineluctable consequence of the nature of Reality or the True One. No creator God is necessary, all would follow 'Tao being what it is'. Al Kindi cannot ground his philosophy on a phenomenon that is in the world of change because this is the world he needs to explain. He reaches back before this to the Unity of the Source, from which perspective the world of change is emergent and not truly real.
To make sense of this view requires studying the meaning of 'Unity' in depth. To me it seems the most difficult word in all of philosophy. It refers to a phenomenon beyond all distinction and division which cannot be said to be 'this' or 'that' in any instance. It is for this reason that the mystics use a language of contradictory complementarity when speaking rigorously about the world. When Heraclitus claims 'We are and are-not' he is endorsing this view and indicating the problem of predication and proposing the Unity of All. We would have to acknowledge the partial truth (thus falsity) of two opposite views (we are and are-not) in order to transcend them.
In order to see why two opposite statements are required for rigour in our statements about the world a study of Nagarjuna's 'Two truths' or 'Two Worlds' would be valuable. It is not easy stuff, however, and some dedication would be required to see the equivalence of the views of Al Kindi, Plotinus, Bradley and Nagarjuna.
It's such a huge topic that justice cannot be done to it off the cuff here. The view endorsed by these philosophers may be justified in logic or even more or less arrived at, but really it is an outcome of experimental practice and direct experience. Al Kindi is leading us towards the Perennial philosophy and the idea that we can establish the facts about the One and the Many, the Origin of this universe and even God without having to read books or visit internet forums.
There is a very useful and afaik trustworthy series of books called 'The Essence of ...' by Eagle Editions. I think one is 'The Essence of Sufism' and this would be a good introduction to the view that Al Kindi is describing. Sufism pre-dates Islam but nowadays is viewed as Islamic mysticism. If he is read alongside Al Halaj and Rumi and the rest of the literature his ideas may seem more clear and well-developed.