"I don't want to appear as a stiff who won't fit in with the culture."

I do not think that just by wearing a suit, you would give that impression. everyone understands that interviews are always a little more formal than the actual job. Even if the startup company where you will interview has dress code which includes shorts or jeans, I suggest you go in a simple business suit. The worst that could happen ? The interviewers will tell you that they are not that formal but thats ok. If you go without a suit, it may not hurt your chances either especially at a startup but why take a chance. My suggestion: go with a business suit. You have nothing to lose with that.