Congratulations on your new Sunbursts, sir! They sound like beautiful specimens and you certainly seem to have done a wonderful job of acclimating them and introducing them to their new home.
Hunting hermit crabs is a bad habit that some seahorses develop. It can be very annoying to the hobbyist when one of his pampered ponies develops a taste for fresh crab meat and decimates his or her cleanup crew, but the habit is not harmful to the seahorses that occasionally indulge in it. Over the years, I’ve had a few seahorses that were confirmed crab killers. These particular ponies were persistent hermit crab predators that specialized in plucking the hermits out of their shells and attacking their soft, unprotected abdomens, and they honed their skullduggery to a fine art. They were experts at extricating the crabs and would eat only their fleshy abdomens and discard the rest. Mind you, that was only a few individuals out of a great many Hippocampines, but I could never keep hermit crabs in the same tank with those specific seahorses.
But you can rest assured that your seahorses unusual diet will do her no harm. Kealan Doyle conducted a study on seahorses in the wild in Portugal in which he did a stomach analysis of wild caught individuals, and was quite astonished to see parts of quite large crabs and shrimp in their stomach contents (Neil Garrick-Maidment, pers. com.). Of course, it’s good that Ocean Rider seahorses are such aggressive feeders, but it is inconvenient at times when they take a culinary interest in one’s hermit crab sanitation engineers.
Yes, sir, you’re 45-gallon tall pentagonal aquarium should be able to accommodate three pairs of Hippocampus erectus. The suggested stocking density for Mustangs and Sunburst (H. erectus) in a system like yours is one pair per 10 gallons of water, so your tank can theoretically house four pairs of seahorses or about nine individuals, although it’s always best not to stock your seahorse tank to capacity. But keeping three pairs of erectus in an aquarium like yours will still allow a comfortable margin for error.
Best of luck with your new seahorses, saltfreak!