I think the 4-line will still eat the live food when he finds it. It's gotta be tastier than the frozen stuff. Besides, even a mature 50g tank with productive fuge might not be able to meet the copepod needs of a mandarin. I'd restrict yourself to mandarin that takes frozen food in the store. Otherwise it doesn't sound like a good idea to me.
Any of the pistol goby combos would be great. I really like lots of small fish filling various niches in a tank. More intersting than four large fish in my opinion. The prawn gobies seem to come in two sizes. There's one group that grow to about 5 inches (huge), like the pinkspot. Then there's the smaller group like the whiteray, blackray, randals. I think the small ones are more colorful and interesting, and also have the advantage of being able to keep a pair if you want. Their paired candy-cane shrimp are also better looking and won't undermine your rockwork the way the 3 inch tiger pistols will.
Nate