As it is pot grown, you can easily leave the plant in its pot for a few days/weeks before planting it. Just ensure you keep the plant well-watered, and it gets enough sun. Don't leave it for too long, however, as you don't want the roots becoming root bound.
Your Wall Cotoneaster should be trimmed twice a year- once at the beginning of June, and then again in late September. You must prune at these times only, as you do not want to remove any lovely, new growing shoots. After they have been pruned, the plant will also benefit from feeding and mulching- use a general fertiliser or any other high-potassium fertiliser.