Real wheel question on trainer.

Get an inline cassette or as close as you can to it (11-21, or 11-23). I built a 13-22 for myself out of other cassettes. It's nice to have one-tooth gear changes on the trainer, means the cadence/effort is a lot smoother for steadily increasing intervals. Then again I have an old school trainer, the newer ones that automatically increase the resistance regardless of your gearing don't probably need to have this one-tooth shifting precision.

Also buy a chain checker if you don't have one already (or just measure chain stretch with a tape measure, google for how to do it). If your chain is ok and doesn't need replacing then you should be fine with any (not worn out) cassette on the trainer wheel as long as it's the same number of speeds and compatible with your shifters and derailleur obviously. If your chain is already past the point of needing to be replaced, then get a new chain at the same time or it will probably skip on the new cassette.

As long as you're not getting a trainer cassette that has smaller gears (bigger cogs) than you're already using, your current chain length and derailleur will be fine.