UHMMM you're contradicting yourself dude. "Most compatible Nix clone" with what, itself? That's not compatibility. That's Microsoft-style compatibility. And yea if you write something on Linux it probably shits itself on Solaris, that's because Linux and GNU in general hold your hand and make your life easier. People take simple stuff like grep -q for granted... I choose Linux specifically for this reason, and yes I love it too dude but try writing your shit on Solaris or FreeBSD, THEN moving it to Linux. Chances are it will work flawlessly. I don't just mean some sh-glue, I'm talking even freaking C code. I've personally witnessed Linux (well really gnu libs) let off-by-one errors happen in run time without segfaulting, Solaris totally says 'fuck you' immediately, and you get to fix your code so that it is actually right. Luckily, I don't worry about mixed environments anymore and I am that asshole that uses tons of bashisms and gnu-specific stuff everywhere.
Hate to get off topic here but, I have to disagree with you on quotas. Quotas are a good thing(tm). "Time Burden" being key here; anything the user can do, should be done by the user. Unless you have a bazillion admins (never seen that before) then the user needs to accept some responsibility for what I consider to be trivially easy.
I really don't care who you are, from peon to CEO, part of your job is proper e-mail archival. Keep things nice and tidy with filters. Set automated pruning according to your requirements. Every so often, delete some stuff by hand, or burn it to a CD.
It's just like any other type of filing. If you were an accountant would you leave a huge pile of bills and invoices on your desk until it was so big you couldn't get to your desk?
Don't panic.