Any thoughts on adding crossfit workouts to the indoor cycling routine over the winter? Just looking for something new to do when it gets cold out. Also - bad idea with previous herniated disc issues?

Crossfit seems to be fairly random. One day you do a bunch of one exercise, the next day it's off to something else. I think that a more focused program would be more useful for cycling. You can address muscle imbalances and work on muscles that support cycling, and try to build strength in the cycling muscles (although there is some question if that'll actually make you faster). See the book "Weight training for cyclists".

If you have back issues you need to be aware of the stresses that various exercices put on your back and avoid them or do them carefully. Sometimes crossfit people get hurt because the trainer encourages them to do too much, or they are unfamiliar with the exercise and it turns out to be one that bothers an existing injury.

Why all the cardio?
Why not bike and lift weights? (This is what I am trying to do now. My idea is I should be able to push higher gears.)

CrossFit can be ok, if you're into what they do. It very much depends on the affiliate.

I would doubt you would be able to safely do much of CrossFit with previous herniated disk issues. There are ways to lift weights with back problems, but CrossFit ain't it. I would not/do not have my clients do some of the things CrossFit does, for orthopedic concerns, and the list of advisable CrossFit activities gets even smaller when there are prior issues.

I am an avid CrossFitter. Been doing it for about a year and a half. It's the only thing thats ever held my interest in the gym.
I'm totally new to cycling and look forward to it increasing my Cardio.

I think the combination of both will be a good thing.