合适 is a adjective. 适合 is a verb.

I'm a native speaker, and I think there is not much different between 合适and 适合。 这件衣服很合适你and这件衣服很适合你are both right. Even though 这件衣服很合适 has no problem in grammar, but I still think this sentence lost the object.

I think there is no difference like in 外国 and 国外。 But I'm not native speaker

这件衣服很合适。 这件衣服很适合你。