In Canada, IFR altitudes guarantee you different minimum terrain/obstacle clearances for various regions and stages of flight. That's anything from 2000' clearance at the GASA (geographical area safe altitude) as marked on LO charts down to 35' over the end of the runway for take-off minima. I don't know of any one place where you will find all the guaranteed/required clearances written down.
See CARs 602.124 for off-airways obstacle clearance rules. Published MOCAs guarantee you the same clearances, but you need to adjust them for cold temperatures.
Reporting procedures for IFR flight are in AIP RAC 8.1 and in CARs 602.125.
Re: FL180: Yes, it's a usable flight level but not in low pressure conditions. If the altimeter setting is below 29.92" then there would not be 1000' between aircraft at 17,000' and aircraft at FL180. See AIP RAC 6.4.3.
Edited because I hadn't answered the FL180 part of the question.
[This message has been edited by Luftwaffle (edited 01 May 2001).]