The issue is: 

I have vector/topo/image tiled maps for some places in north america which were published as ArcGIS Server REST service. These maps (A) are based in EPSG26912 projection. Now I wanna use these as basemap for my openlayer web app; besides, I also want to consume other base map sources (B), e.g., OpenStreetmap, Google Maps, ESRI, etc. Since most of these maps are in EPSG900913 which is sphericalMercator projection to my knowledge, if I just add these base map layers into openlayer map component, the group A and group B won't overlay correctly, which means they are displayed as seperate maps. I know this is something related to projection, and might need some code with proj4js, however, as a newbie, I dont really know how to start with.

Basically, I want to put OSM/Google maps as the base map for the whole world in tier 1, then put EPSG26912 base maps for some places in north america in tier 2, and put other WMS or WFS in the top tier.

do I need to transform EPSG26912 to EPSG900913 or opposite? I guess I should use EPSG900913 as the base projection.

Appreciate any replies!