if you're willing to add that much sand (i understand a lot of people aren't), then you're pretty much gold. You don't have to have a 2ft sand bed until it is full grown really... you could just get away with adding over time. that way you won't have to start with a brand new sand bed at one go. just be sure that you do add sand as needed.
try feeding less at a time to get it to eat the whole silverside. also feed a variety of frozens like squid, cuttlefish, prawn, krill, etc. largely easily found at the grocery store. they aren't picky and hungry enough, will eat just about anything. it would be good to soak in selcon or some kind of supplement once in a while. most mantis keepers general rule is at least 3 different frozen foods, cycling between them.
i guess fully acclimated mollies would be okay, though not ideal. they will never have the perfect nutritional content of a damsel for example. plus the acclimation over the period of months just seems like more trouble than it's worth imo, especially since the L. maculata doesn't need live feedings.
for the peacock, look closely into that as well as a large peacock is one that *can* smash glass tanks. tank breaks are rare but do happen. most tank breaks are on the bottom pane as the mantis digs, so lining the bottom with a layer of acrylic scraps/silicone greatly reduces that already somewhat remote chance. minimum of 30g imo. these guys make U-shaped burrows in the rock. as long as it is separate from the L. maculata, all will be well.
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