Look at Option 1. Fishman says he might put in 3 corydoras catfish and 2 dalmatian mollies. From what I've heard, corydoras catfish and mollies both require a 15 GALLON tank minimum. But, since you've ignored that, I'm assuming 2 mollies are equivalent to 2 corydoras catfish, because we're talking about STOCKING here, and they both require the same tank size. So, 6 isn't a huge jump. Now, I personally do not believe corydoras catfish require a 15 gallon tank minimum. Additionally, Fishman apparently wants corydoras catfish for a 10 gallon aquarium (he mentioned them in both of his plans), so why not? I'm also assuming Fishman can only afford up to a 10 gallon. Furthermore, a school of 6 cories would be do-able if he kept up with water changes. I don't understand why a difference of 3 more fish would make a huge difference. The only reason why I recommended 6 cories was so they would be "happier" in a school, if that's what the OP wants, but if he doesn't really care, I don't see why any more than 3 and less or equal to 6 wouldn't work, especially if the only other inhabitant is a Betta. If the stocking issue is about swimming space, an aqueon 16 gallon bow front isn't that much wider than a 10 gallon; I use the 16 bow front as an example because I don't see ANY 15 gallon aquariums at any LFS's. Again, Fishman may only be able to afford a 10 gallon. Also, what TYPE of corydoras? What about the SMALL species like Corydoras hasbrosus or pygmaeus? If you think a tank bigger than 10 gallons is the minimum for cories, how would you know? Are you a corydoras catfish? Put it this way. Would you rather live with 5 others in a spacious tank, but it has NO plants or hiding places; basically it's just a bare tank, or would you rather live with others in a somewhat cramped tank, but it has many plants, hiding places, and a fine-grained substrate? Again, cost comes into play. 10 gallons is such a popular size because it can accommodate many more fish than smaller sizes, yet is small enough to only have to spend a small amount on supplies.