A hard drive in Idle state uses about 20~25 Watts of electricity.
You can always set it to go into standby after you don't use it for 5,10 or 30 minutes. Its up to you. The only side effect is that when you try and access the drive it must spin up which will make you wait about 2~5 seconds. Also- Do not put swap file on there or program files because then it will never spin down. Lots of spin up and down does NOT reduce the life of a hard drive(don't believe in myths)
Adding a faulty hard drive will cause issues with your operating system. But a hard drive that is in good working condition will not affect any speed. As suggestested you can use RAID- but if raid fails it is can be complicated to recover the files (it depends on what raid you use) as a home user just backup your files to the cloud and do not worry about raid. It has very little performance gain if you don't know what you are doing and data redundancy is easily achieved with DropBox or Google Drive but there are other out there.
You can dual boot from the same drive or use each drive as its own dedicated OS drive. Just remember to backup your files to a place that you can get them when things go wrong with experimenting.