Thanks for coming back to the communities.
If I understand correctly, you have set up Windows 8.1 on two disks with RAID 0 and you have purchased another SSD drive to install Windows 7 on it.
At this stage, you need to start by installing Windows 7 first on your SSD. Whenever you are going to do dual boot, always the earliest operating system is installed first.
Change BIOS settings to RAID. Every time you change SATA modes, you need to reinstall the operating system.