do i need to download el capitan for my new macbook pro 2015?

No, it's not necessary, though a few years later, various programs and websites may drop support for Yosemite, so you would probably want to upgrade to a later version at that point.

MacKeeper is not malware, it is just a scam. No anti-virus software should be used, as many such programs are harmful. If you want a program of some sorts, I would recommend MalwareBytes. It specifically focuses on adware and trojans, the only potential threat to a Mac at this time. MalwareBytes will not prevent adware, as it is a removal tool, but if you do get infected it can get rid of it.

Yosemite is a fine operating system and very stable. Apple and software developers will continue to support it for quite some time. It varies from developer to developer and also depends on the type of program but usually programs continue to be supported for the older versions of the Mac OS for 2-4 years. Historically Apple has released security updates for a couple years. I know lots of people who have made it a regular practice to upgrade their computer a year after it is released figuring that by that time it is stable and ready for use. I like to be on the bleeding edge but I do always keep one external drive with the previous OS just in case. You'll eventually want/need to upgrade but for now there's no compelling reason.

Apple's built in protection does a pretty good job of keeping our computers free of malware but you and I are the weakest link. On my own personal computer I use no anti-malware software and I've never been a victim of malware (though I do run MalwareBytes from time to time to be sure). I stay away from warez & pron sites, download software mostly from the vendor's website rather than collection sites, and delete spam immediately without viewing it. I recommend that you use no anti-malware scanning software and that you avoid so-called cleaning software as well.