Other mods and tools
How T3SDK behaves alongside the other tools Thief: Deadly Shadows players commonly use. Where the repository has no direct evidence either way, this page says so plainly instead of guessing.
Sneaky Upgrade and Direct3D wrappers
T3SDK installs as System\dinput8.dll and only forwards DirectInput calls on to Windows; it never touches d3d8.dll. Sneaky Upgrade and the Direct3D wrappers people run under it (d3d8to9, dgVoodoo2, DXVK) all work by replacing d3d8.dll, so the two don't compete for the same file. T3SDK's own Direct3D hook (for the borderless window) goes through T3Main.exe's import table rather than by supplying a d3d8.dll of its own — see SDK settings and tools and Engine internals. That's why the two are expected to coexist; it hasn't been confirmed by actually running them together in the game.
Sneaky Upgrade also matters for content mods: its override folders, Content\T3\PCTextures\DynamicallyLoaded and Content\T3\MatLib\DynamicallyLoaded, are where most texture and material packs for this game go, and a .t3mod package can ship files straight into them (see Installing mods and Content and texture packs). Those packs need Sneaky Upgrade installed regardless of whether T3SDK is; the mod's own description should say so.
Widescreen and field-of-view patches
WidescreenUI (on by default) re-lays-out the game's menus and HUD for your screen's aspect ratio, but it doesn't change the 3D field of view during play — a true widescreen (Hor+) fix for that hasn't been found yet (see Handoff: next steps). If you also use a separate field-of-view patch, how it works matters: a tool that edits T3Main.exe itself, such as the community's T3FovPatch.exe, changes the very file T3SDK checks the identity of at start-up. T3SDK only hooks the one exact supported build (Steam, patch 1.1); on a file that no longer matches it, it disables itself and leaves the game to run exactly as it would without T3SDK (see Unsupported builds). It doesn't undo the FOV patch, or refuse to start the game — it just doesn't load. Steam's Verify integrity of game files puts back the original T3Main.exe if you'd rather have T3SDK.
ReShade
ReShade wraps the game's own rendering API rather than dinput8.dll, so installing it alongside T3SDK doesn't clash by file name. Beyond that, this project documents nothing about ReShade specifically, and the combination hasn't been tried: T3SDK also hooks Direct3D 8's device creation and reset, through T3Main.exe's import table, for the borderless window (see Engine internals), and whether that gets along with ReShade's own hooks is untested. Treat it as unverified rather than assume either way, and say so if you try it and hit a problem.
The Steam overlay
The game always starts through Steam, whether you click Play in the launcher or in Steam itself (see Playing with T3SDK), so the overlay attaches the same way with or without T3SDK. T3SDK adds no file that Steam itself uses, and it hooks nothing before T3Main.exe is already running. Whether the overlay's own rendering hook interacts with T3SDK's borderless-window and cursor fixes hasn't been checked either way.
Other dinput8.dll mods
T3SDK's own loader is System\dinput8.dll — the one file a game like this only has room for one of. Installing T3SDK when another tool's dinput8.dll is already there does nothing: install refuses to overwrite a file it didn't put there itself, and copies nothing rather than guess which one you meant to keep (see Playing with T3SDK). The reverse holds too: dropping in another dinput8.dll-based tool after T3SDK is installed overwrites T3SDK's own file, outside the launcher's knowledge. Only one such tool can be active at a time; remove one before installing the other.
What isn't covered here
If a tool you use isn't listed above, this project has no specific information about it. As a general rule (not a guarantee): a tool that only adds its own files alongside the game, under a different name, is unlikely to clash; a tool that also wants to be System\dinput8.dll, or that patches T3Main.exe itself, will run into T3SDK for the reasons above. When in doubt, ask in the Taffer Tavern on Discord — someone may already have tried the combination you're asking about.