I am so glad Howie is doing well. And, yes, male Guppies can live with either male or female Betta. It is a much touted myth that male Betta will see them as another Betta and attack. I've had them together for years and the ones that didn't get along with male Guppies also didn't get along with anything else.
In the 10, you could have 3-5 male Guppies in addition to a Betta. It could also have some dwarf Cory for the bottom or a Mystery Snail. They are so pretty with their various colors.
Have you tried frozen for Ari? I feed any ailing fish frozen or live.
Babies are so fragile. I asked two friends who breed Betta for their thoughts. It is not unusual in baby Betta to weaken and not recover. Both also mentioned that freeze dried is not good for them as a steady diet; not enough nutrients. They require live and frozen to do their best. One friend, who no longer breeds, believes two things are why PetCo and PetSmart babies have such a high mortality rate: Improper nutrition and not enough water changes.
Best of luck with Ari; I know what it is like to watch them weaken. But another Moderator, Veloran, says: "As long as the fish is fighting then so will I." That's my theory, too. Then I know I did my best for them.
If your dog thinks you are the greatest do not seek a second opinion