CS 5.1 may run on your mac in 10.11, it may not, chances are if your needs are not to sophisticated you may not have a problem but if you do there are some steps here as to making your system a bit more responsive to having legacy Adobe products running on newer Mac OS's.
As for the two most popular Photoshop alternates (IMHO) they are Affinity and GIMP which you can find tutorials on you-tube for both. Gimp is free and well known in the 3D and game mod community as the go-to software for 2D needs when PS is not an option, it's also capable of some light "illustrator" featuring but I've only dabbled with it.
Affinity appears to be a bit smoother and now makes an "Illustrator" product as well for around $50.00 US. Neither one is "photoshop" (or Illustrator) but they have their similarities. I think they only make Mac software but they appear to be making a name for themselves and Windows users have been asking them for a Windows port
https://affinity.serif.com/en-us/
https://www.gimp.org/downloads/
Illustrator alternatives I'm not versed in but you can check this link for further reading
https://www.nyfa.edu/student-resources/six-free-alternatives-to-adobe-illustrato r/
if you have specific questions it's best to start a new thread to discuss further.