I've put sps in a tank that was a few days old as well, now it was set up with real live rock, straight from the ocean and kept submerged, and had a huge system volume so I wasn't too worried about stability issues. One issue with new systems is a lack of food for the coral as well as stability. If you don't have alk swings and have a few fish to fed a little daily then you should be ok. 6 months is a long time to wait and isn't entirely necessary. I would say quality live rock is most important on small tanks. It should have a decent amount of coralline and some sponges ideally, showing that its more biologically active than just ceramic filter media like in a FW tank. If your completely new then 2-3 months is a good idea to wait to manage algae issues that occur and start with cheap sps, they tend to be hardier and grow faster.