Sorry I’m a bit late to the party, but here’s my take:
It’s best to keep it as simple as possible, especially when launching a new portion of the site. New additions can always be made, yeah? While sub categories are always fun, getting too specific can make it overwhelming for new users and leave them rather neglected.
Solifugae is a fantastic order, but to be honest I had never heard of it and I’ve been in the game for a while. There may be a handful of keepers but not enough to mandate an entirely separate category.
A general category for spiders would be best at first in my opinon. While they are classified differently as “true” spiders, people will know to look for spiders under, well, spiders. There is a strong tarantula community and a jumping spider community, but not much in the way of other spiders - others are typically wild caught and kept in captivity, not bred for sale.
Assassin bugs would make a good separate category. I see them frequently listed for sale, brag posts on forums, and on display/available at shows.
As pointed out already, an isopod category would be valuable- I am a bit biased since I am eyeballs deep in isopods, but there are many dealers, interested parties, and the isopod market continues to grow with the bioactive movement.