Both C and U, mostly U.

@all: thanks so much for helping me!

You can use all of those. You have a lot of flexibility with uniform as a noun. You can also use, as Paul indicates "They are in uniform."

Rather than 'I'm wearing uniform', you would say 'I'm in uniform'. For example, we refer to 'a police officer in uniform' to distinguish them from a police officer in plain clothes. Saying that someone is 'In uniform' will be understood to mean that they are wearing uniform. Your confusion may be because 'uniform' is a noun (with the meaning of a type of clothing outfit) derived from an adjective. As an adjective, it really means uniform outfits of clothing, ie outfits that have 'one form', or a single style. (I'm sure you will know that uni- means one, as in University, unicycle, and unicorn). So its countable when used as a noun, but uncountable when used as an adjective. In your second and fourth examples, its used as an adjective, with the noun (clothing, or outfit) implied.

Uniform without the 's' at the end gives the sense of a unitary dress code. With the 's' is also a dress code, but shows that there are different types. The word is almost always used with a modifier (a, the, his, her, my, etc.).