205.2a The card types are artifact, creature, enchantment, instant, land, phenomenon, plane, planeswalker, scheme, sorcery, tribal, and vanguard.110.1. A permanent
Don't mix with "creature type", any non-artifact creature with CMC<4 works well with your combo and one of each creatures listed is sufficient.
By the way you are allowed to return an artifact creature to you hand if a creature enters the battlefield. 205.2a: You may return the bold ones to your hand when one of the boldunderlined enters the battlefield. Tribal is a little tricky since 300.2b Each tribal card has another card type. Casting and resolving a tribal card follow the rules for casting and resolving a card of the other card type. you have to consider the permanent type. [Edited by knarf_the_dwarf on 27/Mar/13 at 4:12AM]