My plan is to stay in Ljubljana for eight nights and take bus or train on day trips to Bled and Piran. Is this practical? How about with a rented car in early November?
These places (among others) are best done with an overnight, IMHO, not as a day trip. They're quite easy with a rented car (Driving in Slovenia is great, by the way).
Where else are you planning to go?
I really liked Slovenia, and also like Ljubljana. But 8 nights in Ljubljana seems like about 6 nights too long to me (unless you have an extremely long trip and/or you've already been everywhere). The region has so many great places to see, it seems to me that you would probably be missing out on lots of them (including parts of nearby Croatia) if you spend 8 nights in Ljubljana.
One caution I'd add: in early November, the mountains will have snow, so if you're planning travel on mountain roads, take that into account.
We talked about Bled and winter driving in your other thread. As I said there, it may or may not be snowy by early November, but if it's not, I recommend the scenic detour to Bled via Skofja Loka and then the especially scenic part via the mountains - Jamnik, Kropa, etc. Otherwise, I prefer Bled as a day trip unless I'm exploring the nearby areas like Lake Bohinj, for which Bled makes a convenient base. But I prefer Ljubljana at night to Bled at night, personally.
Piran I'd guess would be a super easy day trip by car in early November - mostly on the expressway. Play it by ear with the weather. It is possible to book cars and cancel/change them at the last minute. Book a car and if it snows the night before, cancel it. There are also direct buses to Piran, FYI.
Eight nights is kind of long for Ljubljana, even though I love it. You might consider a day trip to Zagreb (train) for a few nights - very easy to do. Put a few day trip options on your agenda and do them if you feel like it when you get there, and be flexible with the weather.