Understanding multi arch

[Itay <debian@itayf.fastmail.fm>]

Hi,

My system is amd64:
	$ dpkg --print-architecture
	amd64

I wish to install acroread from deb-multimedia [1]. 
According to the FAQ I should do
	$ dpkg --add-architecture i386		# (*)
	$ apt-get update
	$ apt-get install acroread

My understanding, based on the manual [2], is that even after 
executing (*), I should still have the same old "pure" amd64 system 
as before, decorated with some i386 "islands".
The latter will be confined to those i386 packages (and their 
dependencies) that I would explicitly install by
	(a) requesting a package that is available for i386 but not
	for amd64 architectures (such as the case with acroread);
	or,
	(b) by explicitly requesting -- for whatever reason -- to
	install an i386 version, e.g.:
	$ apt-get install <pkg-name>:i386	# (Correct?)

The bottom line --
Can some knowledgable people confirm the above anticipation, or 
point out some obvious pitfalls that I missed?

Thanks in advance.
Itay

-------------------------------------------
[1] http://deb-multimedia.org.
[2] https://wiki.debian.org/Multiarch/HOWTO