they are probably all the same basic strain, but with guppies, even with good strains you need to cull undesireables if your goal is an all look alike strain. they have recessive gene's that pop out in some of the fry.
i do some research, but take everything with a grain of salt. so many varied experiences, and so many old wives tales in fish keeping. you kind of have to sort through the bull dung.
options really become dependent on the betta's temperment. pleco should be fine with a betta, and of the other two options id lean towards the rasbora as they are just a bit hardier.
nothing, just long time use. key to any scaping is to use what is most pleasing to your eye. slate is good for making structure, and even screwing down pieces of wood that wont stop floating.
stuff with copper sulfate in it will generally kill snails quickly. to be noted, it will kill any snail,shrimp,crab, it doesnt care. also in heavy enough doses that are sometimes required to eliminate snails, it can harm fish.
that stuff is spooky first time you get it. its an ohhh crud whats happening moment. it will go away in time. some pieces of wood take 2 or 3 weeks, some take a couple of months. its ugly but harmless.