First, groundcovers won't eliminate weeds - they only retard their development. And some are much better at it than others. For underplanting large conifers, pachysandra is a good choice - it is quite tolerant of dry shade, which is the usual planting condition in this situation. But go with Pachysandra terminalis rather than the native species (Pachysandra procumbens). First, it is fully evergreen and it spreads faster than Allegheny pachy. Both factors will be better for weed control.

Are the needles an applied mulch or are they shed needles from the spruce (in which case they'd be spruce needles :-)) ? If a mulch, just rake them aside to plant, then replace around the new groundcovers. No particular tricks to planting and no need for added soil or mulch - the conifer needles serve that purpose admirably. The ground beneath the trees will be very rooty and hard to dig so start with small plants. Plant in a zig-zag pattern rather than in straight rows 12-16" on center (use the closer spacing if you want faster coverage). And water well. Even plants suited to dry shade will need extra water until they get properly established.

Great info, thanks. The needles are not mulch, but spruce needles from above. I do have the spruce disease that is killing a lot of trees in Michigan. Do I need to remove these needles before planting?