Many spiders are solitary predators.
Many spiders, especially orb-weaving spiders, basically "min-max" evolutionarily by putting "all their character points" into being able to devote around a third of their body mass to silk and making elaborate web designs, and as a result, have very weak legs, are generally slow and clumsy, have poor eyesight, and actually can't tell the difference between a fellow spider of the same species and some prey that flew into their web.
Many "sexual cannibal" spider females actually aren't purposefully eating their mates - the female just couldn't tell it was a male spider of her species here to mate unless the male was plucking her web (they play web strings like a musical instrument to signify they are there to mate) and if he takes too long, she forgets he's there to mate and decides he must be prey.
As for the spider colonies like bee colonies, there are spiders that are
communal web-builders. Check out the
200-meter wide spider colony cobweb. That was made by normal, non-giant, real-world spiders, and it's basically enough to cover an embark. Harvest that.
Finally, Toady basically rush-jobbed many of the donation creatures because he simply wanted to get them all in within a year, and couldn't spend a month on every creature. That's why he basically relied upon using the same code from the spiders he already had.