Which actions give stress depends on whether you have succeeded in proposing intimacy at least once and which sexual traits the slave has, so there is no easy way to know exactly which actions will give stress. Without traits, anything in S&M, milker, face sitting, order groveling or masturbation, footjob, and anything involving tails.
While the game allows for orgies, it doesn't really support them. The sex system was left partially complete. You can use the constants mod to increase the number of turns for sex by changing "timeforinteraction". Or with a decent text editor you could add these lines after line 3935 in Mansion.gd(2 tab, 3 tab):
if sexslaves.size() >= 4:
get_node("interactions").turns += sexslaves.size() * 5
If you select a slave and click Statistics, then it will show all relationships with that slave on the right.