Before I did anything, I read all the info on Simswiki.info to keep myself straight. As per the instructions, I installed the nointro.package and nobuildsparkles.package, and they both worked properly. After that, I tried to install some custom content .package files. Trying to keep things organized, as advised, I opened the Mods\Packages folder and created a subfolder path Clothing\Tops\syn28_MTS, with the last folder designating the file creator and associated website. And of course, I stored the CC files in said folder.
When I tried to start the game, the startup process began properly, and I got past the screen with Reticulating Splines and so on. Next came the screen where I select which group of Sims I want to play with. I selected my people, pressed the check mark button, and the next load screen popped up, the one with the second progress bar. The startup stalled out and the progress bar never moved past the start point.
Trying to troubleshoot, I first moved the CC folders to the desktop (taking them out of the game). At that point, the game started properly. Next, I moved one single file of the CC back into the Packages main folder (right next to the No Intro and No Sparkles files). When I tried to start the game again, the same trouble came back.
Obviously, I could have bad CC files... but assuming that they are good files, is there anything I may have done wrong with my process?