Check: Systemd VS Sysvinit - #14 by triantares , specially the last comments with the results of a test
Also, anybody wanting to do tests on an installed system to betatest replacements are welcome 
No, just the same way as you can do it in debian (following debian howto's), but note also that different versions of kernel would have a different structure of things, I mean, things that changes, and so uncompatible with different versions of applications / system
Steam included in the next alphas, already tested here
About the virus topic, this would need some investigation, unfortunately this sentence causes controversy, but basically a virus is a piece of software which replicates itself and contagies other computers, this is simply not possible in linux systems because of how the system works (you know, the system is well architectured and you need permissions on the system to do specific things, etc, unlike windows where is just a false idea about a system with permissions), NOW, if you are able to exploit a bug in the system and reach root access and similar things we are talking about another thing, we are talking about an issue in the system, a bug, which eventually can be exploited to reach a goal, but again, a bug is a bug, if you have a bug in your filesystem it can also destroy all your data, or make your cpu burn. Basically that sentence could mean that a linux system is inmune to viruses just because of how the system is made / structured / designed (period), while as comparing to other systems (windoze) that has not a security structure that prevents specific things to be run. (and again, a bug is a bug, and can happen anything... except space time traveling, probably)
But how to describe all this virus topic in a single, short and concise sentence?
BTW, your idea about "for multimedia", "for security", etc.. specific purposes doesn't sounds too bad to be implemented for the installer, I mean, not possible for the live mode but for the installed it can be as simple as a selection of extra packages to install, whereas debian (which is based) is probably the kind
I mean, since there's a few users that could have specific needs for the OS, could be nice to allow them to being able to have these features 
for example @triantares with the example of QubesOs, if he's able to setup this system on Elive 3.7.x, could be replicated selecting this install profile from the installer, and even if is a slow process it can be amazing to have these things in one shot
Alright, its simply not possible to include it "as default", only as an detached "extra"
I just wrote this draft suggestion Installation profiles, full modularity, specific purposes 
