I was only going to post the question but I ended up lifting some weights with my liver for science, and I think I came up with the answer (if someone knows anything else please do tell).
The answer is:
1.- There are chem (aka aid items in general) classes. For instance, wine and whiskey are both alcohols, and normal mentats and orange mentats are both mentats. I do not know all chem classes.
2.- Chems from different classes stack, fully.
3.- Chems from the same class do the following: if each chem changes different stats, both changes are kept. If the same stat is changed, the effective value is that of the last chem taken, meaning order matters. I don't know yet if this also affects the length of the effect.
To illustrate, I'll show the results of the experiments I made, chems shown in the order I took them:
Experiment 1:
Reminder of the effects of chems used:
Buffout -> +2 STR, +2 END, +50HP
Mentats -> +2 INT, +2 PER
Bufftats -> +3 STR, +3 END, +3 PER, +65HP
Orange Mentats -> +5 PER, +10 accuracy in VATS
S P E C I A L
6 5 4 5 6 7 2 -> initial stats of my character
9 8 7 5 6 7 2 -> bufftats
8 8 6 5 6 7 2 -> bufftats + buffout (buffout overwrites strength and endurance. Perception from bufftats is kept.)
8 10 6 5 8 7 2 -> bufftats + buffout + mentats (bufftats and mentats stack)
8 13 6 5 8 7 2 -> bufftats + buffout + mentats + orange mentats (orange tats overwrite mentats but not bufftats, so 5+3. Int from the mentats is kept.)
8 10 6 5 8 7 2 -> bufftats + buffout + mentats + orange mentats + mentats (mentats overwrite orange tats)
Experiment 2:
Beer -> +1 CHR +1 STR -1 INT
Whiskey -> +2 STR -1 INT
Taking nothing: +0 CHR, +0 STR, +0 INT
Drinking whiskey: +0 CHR, +2 STR, -1 INT
Then drinking beer: +1 CHR, +1 STR, -1 INT
Then whiskey again: +1 CHR, +2 STR, -1 INT
So remember to take your meds in order. I'm off to invest in addictol now.
Best Answer
If they have, I certainly haven't noticed.
By the time the new survival mode had come out, I had already made a character with the express intention of exploring every building in the Commonwealth. I didn't have a checklist to work with, but I completed that goal to my satisfaction prior to starting Survival mode.
I have not noticed any new beds anywhere.
That being said, there really are quite a few beds around, and you can always make some in a nearby settlement. As long as you don't mind losing your adrenaline, you can usually find a bed to sleep in fairly near your goal.
To illustrate this, Vemonus has provided a link to a Reddit thread that shows the locations of all the beds in the Commonwealth. I can't independently verify that this is 100% accurate, but it gives you a pretty good idea of how common they really are.