▻ TEXTS THAT CAN TAKEN WITHOUT PREPARATION OR CONTEXT
Some texts are virtually stand alone. For instance you can read Plato's Republic and gain a lot from it without knowing whom he was answering or what were the influences on his thought at that stage. A fuller context would illuminate but is not necessary to make pretty good sense of the text and to be intellectually stimulated by it.
You do not need to read Plato, for the theory of Forms, before reading Aristotle mainly because, though Aristotle rejected the theory in its classic version as presented in the Phaedo and Republic (Phaedo 95e-102a, 246a-249d; Republic, VI & VII), very little of Aristotle is centrally concerned with Plato and his theory of Forms. Aristotle does criticise the theory (e.g. Metaphysics, XIII (M), 4-6 and Nicomachean Ethics I.6) but we was an independent philosopher of the highest order and Aristotle's quest for the nature of things is conducted under assumptions that Aristotle does not take from Plato. (But you do need to know Plato's view of moral failure, or weakness of will [akrasia], before reading Aristotle's account of the question in Nicomachean Ethics, VII.1-10.)
The same just-jump-in principle applies to Descartes' Meditations : Descartes' aims and method of argument are clear without any knowledge of the contemporary or earlier philosophical context. The context will aid a fuller understanding but I defy anyone to say that they cannot follow the scepticism of Med. 1 and the first defence of the cogito ('I think, therefore I am') in Med. 2 even if they had never read a philosophical text before.
I think the same is true of Locke's Essay concerning Human Understanding. Context illuminates but a pretty clear basic understanding is perfectly possible without it. Essay, I contains a sustained critique of innate ideas. Many suppose this is a polemic against Descartes but Descartes' position and Locke's critique largely fail to mesh. It is quite likely that Locke's attack on innate ideas was directed as much against the Cambridge Platonists as against Descartes - if not more so. But to a beginning reader, it doesn't matter. The logic and cogency of Locke's critique can be assessed regardless of its target.
I should postpone the Summa if only because of its vast scale but there is much in it that is self-standing - capable of comprehension without immersion in the full context of Scholasticism.
▻ TEXTS THAT NEED PREPARATION AND CONTEXT
By contrast there are philosophers whose work is so intertwined with that of others that their texts need to be read in a certain order. Spinoza's Ethics is hard to understand if you have not read Descartes. Berkeley is incompletely intelligible without Locke.
Kant represents from one point of view a synthesis or creative reworking of the traditions both of Rationalism (from Descartes onwards) and of Empiricism (from Locke onwards and especially Hume). Much is lost in a reading of Kant that is uniformed by a knowledge of these earlier traditions.
Hegel is hard to grasp anyway because of the density and complexity of his ideas and his mode of presentation but he is all the harder if you do not have an extensive knowledge of Western philosophy on nearly all of which he draws. Schopenhauer emerges from the background of Kant and Hegel, without a knowledge of whom the full impact of his work is lost.
All philosophical texts in the Western tradition since Plato are intertextual; they cross refer, sometimes explicitly and sometimes tacitly. You cannot prepare adequately for this. Also all texts have contexts which disclose much about their purpose and methods. None of the remarks above is meant to deny this. But some texts are virtually stand-alone, at least for the purposes of preliminary study, and if you adopt that rule that before you read Y you must read X lest you misunderstand Y, you will find (defeatingly) that it's never the time yet to read anything.