Having the applications installed on the SSD makes launch time shorter. That can save tens of seconds, which is actually significant if you’re bouncing between applications a lot.

There’s no harm in having the cache on the same SSD as your applications, if you have the room. The main principle to keep in mind when distributing things across disks is that you try to avoid multiple things being written to and read from the same disk and across the same bus at the same time. The application bits are read from the disk at program startup, but the cache bits are read from disk during application use, so these two things don’t conflict with each other.