When you mount a file system at a point in a file system, the previous contents become invisible for the duration of the mount and reappear when the mount is absent. They do consume space on the file system though invisible.
The question thread you quoted seems to have a lot of very helpful information. Maybe I can add a little information given your questions.
The contents of tmpfs file systems, as you mention, are not persistent--they always lose their data when the system is restarted.
While I know of no drawback for storing /tmp in memory (except for the memory it consumes), the FHS (Filesystem Hierarchy Standard) says that /var/tmp should survive reboots, so that shouldn't really be a tmpfs (mine is empty at this moment). /var/spool stores print jobs, and possibly other things. /var/log contains debugging information which is of no use unless you have a problem, but then may become very important. I wouldn't put any of these last three on a tmpfs, though I admit that the logs get a lot of writes.
I'm not sure, but I think perhaps the aufs mount is meant to allow one to read from /var/cache/ while actually storing anything you write to it to /tmp/ (which would appear in both /tmp and /var/cache/). I'm having trouble matching the fstab entry you mention in your question with the man page documentation I get with aufs-tools version 1:3.0+20111101-1ubuntu1. I wish the article you quote had more information about this particular item. If you decide to use aufs and have trouble with the syntax I'd suggest you try Andrew Ferrier's fstab entry from the comments in the question thread you quote as it seems to better match the documentation I was able to find.
/var/cache/apt/archive is used to contain .deb files for packages you have installed. If your system is like mine you will find a lot of the space here consumed by /var/cache/apt/archives. You can clean it out with
sudo apt-get clean. A cache is useful for performance, but is supposed to only store replaceable items. The /var/cache/apt/archive cache allows you to reinstall versions of packages without having to download them again. If you want to reduce this space with
apt-get clean you would need to do that when there is no mount on top of /var/cache so the data can be seen and removed from the device and that space recovered.
The noatime option and related nodiratime option give up having the file system remember when a file is accessed (read), with the benefit that there are fewer writes to the file system, improving performance and reducing wear on the SSD. If you care about searching for files that have been read in a certain time frame you would lose that abiility. I don't find myself doing that.
I see some suggest setting an option on your ext3/4 file systems to turn off the journal, which constitutes a fair number of writes, at the expense of recovering some data you have recently written if your system goes down.
If you look here, oldfred gives two commands that can be used with SSD's in post #2.
sudo tune2fs -O ^has_journal /dev/sda1
sudo tune2fs -o discard /dev/sda1
changing /dev/sda1 as appropriate, of course.
The first turns off the journalling I mention above. The second tells the kernel to communicate the deletion of file information to the SSD's controller so it can more optimally manage the device, giving better performance.
My Humble Opinion
I think I would start with using tmpfs for /tmp, use the noatime mount option, turn off EXT4 journaling and turn on the discard option.
I agree with JR0cket and definitely wouldn't put a swap file on the SSD for fear of shortening its useful life.
I've made these changes to my flash drive and, due to the journal change, it has had to take a few more seconds to fix up my file system on boot, having had to reboot at least once. When I reboot or halt apparently the file sync hasn't completed before the system has halted or rebooted and after the last write to the file system. That hasn't been a significant problem for me and I haven't noticed any other repercussions from the other changes.
I hope someone who has done this before or who knows about aufs will contribute more details in answer to your question.