Oil containment is a requirement in the US under the NEC, but it is not a requirement under the current Canadian electrical code. You should check for requirements under the various provincial environmental laws. IEEE 980 also have some information. We have found it is generally driven by insurance requirements more than anything.
For the last few projects, I've designed the containment for 110% of the oil volume and a 24 hour storm event. We've also used a product supplied by Albarrie (sp?) called Sorweb. This is a material that allows drainage of water, but forms an impervious gel when subjected to transformer oil. Containment pits are filled with a 'firestone' (limestone aggregate) which has a large void ratio.
Fire separation between transformers has used a 2 hour metal clad barrier, filled with mineral wool insulation. Roxul has a tested assembly.
Dik