I dont think ive ever let them hook up, despite the game's scripting heavily implying this is what should happen. I stick to canon things where possible, and I dont know if these two canonically hook up. I do know that Don canonically leaves Cassandra, his own household is scripted to have him carry on romantic pursuits all over town, despite his engagement; all that is lost if if he actually accepts the marriage on the Goth's lot (in my game, I had to reset multiple times, he kept NOT running away. "Don Goth" looks so very wrong!)
In my last proper attempt at running Pleasantview longterm, before my harddrive got wiped by someone, I had Darren and Cassandra still friends, but I didnt want to force anything; sims can fall for pretty much anyone very easily. I was only going to let it happen if they actually did things themselves. Theres also the matter of Darleen; Darren is quite clearly haunted by her death, and having Darleen playable in Lucky Palms, where she gets together with Darren and has a happy life until her death, makes it all the more sadder. Resurrection exists in the game, and I think he would jump at the chance to bring her back. (Nevermind the fact that due to muckups in the code before release, the Darleen in his graveyard has the wrong relationship values with him, and they wont be so loveydovey with each other post resurrection unless you fix those values with SimPE. There is a seperate deleted Darleen in the code that has the real relationship values, they were quite high I beleive!)
