On Aristotle's hylomorphic ontology, form and matter are 'principles' or ontological factors involved in the analysis of sublunary primary substances. These factors are not substances in their own right. Now Thomas is an Aristotelian in ontology. But when it comes to God and the soul he goes Platonist. God is forma formarum, the form of all forms, and yet self-subsistent. The soul after death is capable of existing in separation from matter while it awaits the resurrection of the body. Anima forma corporis: the soul is the form of the body. But in the human case the soulic form is more than a principle invoked in hylomorphic analysis. It is capable of existence independent of matter.
The sublunary Aristotelianism and the superlunary Platonism exist together in a certain tension. Whether this tension gets the length of a contradiction is a further question.
"Get the length of" is a classy phrase which has long languished in desuetude. I resurrect it from the writings of F. H. Bradley.