Chronological Order
Ilima: Totem Pokemon (Gumshoos in Sun, Raticate in Moon) is level 12, so I'd expect the regular Pokemon to be around 9 or 10.
Hala: Mankey, lv. 14. Makuhita, lv. 14. Crawbawler, lv. 15
Lana: Totem Pokemon (Wishiwashi) is level 20, so regular Pokemon 16-19
Kiawe: Totem Pokemon (Salazzle) is level 22, so regular Pokemon 18-21
Mallow: Totem Pokemon (Lurantis) is level 24, so regular Pokemon 20-23
Olivia: Nosepass, lv. 26. Boldore, lv. 26. Lycanroc (Midnight Form), lv. 27.
Sophocles: Totem Pokemon (Vikavolt) is level 29, so regular Pokemon 25-28
Acerola:Totem Pokemon (Mimikyu) is level 33, so regular Pokemon 29-32
Nanu: Sableye, lv. 38. Krokorok, lv. 38. Persian, lv. 39.
Hapu: Dugtrio, lv. 47. Gastrodon, lv. 47. Flygon, lv. 47. Mudsdale, lv. 48
Kommo-o: Totem Pokemon (Kommo-o) is lv. 45, so regular Pokemon 41-44
This might not be 100% accurate, but I think it's pretty close.
All Pokemon listed are in their Alolan forms, if applicable
Source: http://bulbahandbook.bulbagarden.net/pokemonsunmoon/guide/island-challenges