Since you probably don't want unRaid to clear the disks, you would issue a New Config and reassign everything accordingly.
That being said, SSD's aren't particularly recommended in the array as you won't be able to trim them and the write speed would then be limited to the speed of the parity drive
Small HDDs would be OK, but if it's old / borderline, then it will impact the ability to rebuild any other drive since a rebuild requires all other drives to be fully functional.
Personally, for downloads I wouldn't care about them being in the array or not. If the drive dies, then just redownload it again.
For appdata, either create a cache pool with multiple matching SSDs for redundancy, or back up the data on a schedule (appdata backup plugin)