Tarski includes a number of theme hooks which let you add custom content to various parts of the Tarski theme without having to hack your changes into the Tarski theme files themselves. Importantly, this lets you upgrade without fear of losing your customisations.
The Hooks Reference is a comprehensive document listing all Tarski’s actions and filters. If you want to make something work with Tarski, read this before you start trawling through the source code, as it will probably save you a fair bit of time and effort.
Writing your own plugins to interact with Tarski isn’t terribly difficult, and you can get a good idea of how it works by reading through this article on adding custom WordPress hooks to a theme. The section titled “Tweaking with a plugin” explains how to add custom actions (like including bits of text or integrating other plugins) to Tarski’s hooks. Another example can be found in this forum thread.