Costume Jewelry with Engagement Ring

It's not really my style, but I don't see why it would be "bad."

I don’t think it’s “bad” but depending on the material, the ring may not be durable. When you say “costume jewelry” I’m thinking like, mixed metals, and that tarnishes FAST. Personally I wouldn’t bother getting a ring that wasn’t 10k or 14k gold just because I know it wouldn’t last.

I personally wouldn't go too "costumey" but if that is what you want, go for it!

What is a 1/2 engagement ring?

Metals have different levels of hardness and durability depending on what they are and what they are alloyed with. Precious metals like gold are still relatively soft and placing a precious metal ring of one kind next to a different metal ring may cause damage to your e-ring. I wouldn't risk it.