Yes you can backup as many folders or even servers as you like within one backup job to one tape, or you can have multiple backup jobs using the same tape, assuming there is enough space on the tape to hold them all obviously.
Just make sure if you choose to use more than one backup job that all jobs after the first one are set to append to the tape not overwrite it.
You need to run a full backup at least occasionally, since if you use incremental backups you will need the last full backup, and then each incremental backup from that point onwards. If you have enough tape capacity (which from your description it sounds like you do), and enough time in terms of backup window, then I would suggest sticking to just using full backups, as incrementals make the whole process even more complicated when you have to do a restore.
