If I install Deepin 15.something, will it detect other OSes on the HD and do the needful to GRUB, etc etc?
While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. (By Stackoverflow.com !)
Most Linux OSes use Grub to boot. Grub will detect any OS currently sitting on your hard drive unless your system is using UEFI in which case you have to disable secure boot or install a secure key with your Linux OS. Grub will not detect an OS this is using UEFI partitions or an OS whose boot code resides on another partition that it can't read.