Can I add external hard drive to start9 server?
Yes, but it would need to be in place during the installation of StartOS. Once installed, there is not currently a way to expand the data drive horizontally. There is a process to move it from one drive to another though (the same process that is used to upgrade your server to a larger drive).
Can I limit the blockchain I am downloading somehow like don’t need all of it right can I say last 200GB of blockchain?
I haven’t used it myself, but there is the option to enable pruning under Config > Advanced