Hello!! Long time lurker first time poster.
I have had my lovely lovely bun, Buddy, for nearly 6 years now and he is quite honestly the best thing that ever happened to me. 
Recently I took in a cat that needed rehoming (Bran, 3) and he’s great… to me!
At first they were good with each other, mainly curious and lots of sniffing as I introduced them through a cage and Buddy was very quickly relaxed around Bran, his behaviour hasn’t altered at all.
However when I let them out together for the first time I noticed that Bran was being semi-aggressive towards Buddy, something obviously I won’t stand for. It was mainly chase-play and a lot of rolling to his back. Buddy was fine and didn’t express any fear but this is behaviour isn’t something I can encourage at all, so since I have been keeping the cat out while Buds exercises. I would like to have them around each other without worrying about Bran’s claws and teeth. 
So really I’m asking; what should I do? Would you suggest a spray bottle? Or am I best just keeping them apart for good? I never plan to leave them supervised but I would love to give Buddy back the freedom he had!! Bran is a lovely cat but and I would really love for them tolerate each other a little better. 
Any advice at all is greatly appreciated, thank you!!!!!!!