Can TDS be played in windowed mode at a smaller resolution (800x600, yeah I know) without using a mod? I don't have a second monitor and need to be able to Alt+Tab out of the game or click off the game to a separate window. I'm running Windows 10.

Idk about that. Have you tried the unofficial patch? It's called "Sneaky Upgrade." When you use the installer for the patch there is an option for mods. Just un-check the boxes for the mods and play with the patch alone. I never tried playing in a window smaller than 800×600 using the patch, but it could help.