Dave!
Hang in there bud, losing fish sucks, it made you mad and you overreacted, everyone does it! nothing I can add will really alter what everyone else posted here, BUT!!! Smart phones didn't exist 30 years ago and the fish were expensive back then too, so we had to rely heavily on the LFS recommendations. There wasn't Google to learn every aspect of the available marine life at that time. You read books and trusted LFS sales folks. With that said, it is obviously much easier to know what you are wanting to add to your tank in this era, but it is nice to trust those LFS and I feel where you were coming from. Those fish aren't very expensive fish and I doubt the guy wanted to make a buck off of you with low priced fish, but who knows.
As far as what happened to them, you may have had an ammonia spike and it may have contributed to their deaths, but... in my experience with both species over the last 30 years, you starved them, unintentionally. Way too soon to add those guys and even if they were picking at the rock (scoorter) and sifting through sand, they were not actually obtaining any nutrition. The goby requires micro fauna that takes months to develop and the blenny has a voracious appetite and can easily strip the pods from a 60g tank in a few weeks. The goby can be trained to eat pellets though, but the blenny is much more difficult to acclimate to frozen foods.
Do yourself a favor though and just dial back your fish purchases and allow that tank to develop a nice large pod base and even various types of micro fauna in the sand before adding anything else, then maybe get one or the other but I would likely stay away from the blenny or a Mandarin in a 60g unless you wait for 6 months or so and maybe have a fuge to recycle new pods.
Don't throw in the towel though, do some research on that coral that is showing STN and make that your goal in life to bring it back! Good luck man, hang in there!!!