Fixing pictures to plasterboard wall in new build

Minimum thickness for plaster board would be 9mm thick so Fischer dry wall fixings would be best. I recently used these type of fixings to fix 2 large mirrors each weighing 20kgs. As for trying to find electrical wires, an electrical detector would be best. Also make sure you drill away from electrical plug sockets and light switches, the wires normally run vertically in most homes but always be careful when drilling into any walls.

The minimum your walls would be 9mm so these will be ok, toggle fixings are good for heavier loads but for the weights you've said would be OK on these also. With any drilling into walls, there would always be a chance of hitting electrical cables, and I'd suggest using a detector to minimise the risk especially with plasterboard walls. Otherwise just check both sides of the wall for any sockets and switches and avoid that area and directly above / below it as this would be where the cables will run. If in any doubt you could get a quote from a local handyman to fit them for you, shouldn't cost much for a few fixings.