Toad,
ULS and SLS are equivelant to Strength Design and Allowable Stress design respectively here in the States. Canada, as far as I'm aware, no longer accepts allowable stress design (not that many trades still use this). There is some discussion in the code regarding which State applies to...
Use isolator material between the steel and aluminum with stainless steel fasteners only. Rubber or vinyl washers will work. For larger areas a cut to fit piece of EPDM works as well.
Don't forget to include isolation from dissimilar metals to prevent galvanic action. Stainless steel fasteners are a must. Design per Aluminum Design Maunual 2005 or 2010.
