"is it possible to do both types of disk in the same capacity tier"
vSAN Disk-Groups can only have one cache-tier device, thus in a hybrid configuration you can only have one SSD as cache and HDDs as capacity-tier
"can I create a policy that just points to SSDs?"
No but you can set Flash read-cache reservations in a Storage Policy to reserve some cache for reads:
thank you very much for your answer, I am very happy with your reaction on my questions.
have a nice day