 Re: meaning of idiomatic expressions
These aren't so much idioms as collocations -- words that usually belong together. Most of them are collective nouns.
A collective noun is a word or phrase that refers to groups of things. English has very many different collective nouns for different things: for example, we talk about a herd of cattle, a pride of lions and a string of horses. A lot of collective nouns have become old-fashioned and aren't used so much now, but it can still very confusing to a non-native speaker.
A few notes about your selection:
1. A "shower" of rain is usually fairly gentle and lasts only a for a short time -- a few minutes perhaps.
2. Are you sure about this one? Usually, a sheaf is a bundle of cut stalks of grain or a bundle of paper. I have found the phrase "sheaf of rain" in one poem ("...Moments ago / A sheaf of rain, weighted with promise, breached / the foothills..."), otherwise I've found it in contexts like "sheaf of rain checks" (a "rain check" is a ticket to a sports event if the event you paid for is cancelled due to rain) and "sheaf of rain-wrinkled paper".
7. A "bunch of keys" is lots of keys all attached to the same key-ring.
9. A "heap of stones" is not really a true collective noun; a "heap" is just an untidy pile.
10. A "galaxy" is a large group of stars (a few billion usually). The galaxy we live in is called the Milky Way Galaxy; our nearest neighbour is the Andromeda Galaxy.
13. I think that should be "grove". It refers to a small group of trees, or an orchard. You don't need to say "of trees", because a grove can only be trees.
14. A regiment is a military unit of two or more battalions; a battalion is usually made up of three or more companies; a company is the lowest administrative unit of an army and consists of two or more platoons and a headquarters. Note that the US military no longer uses regiments, but the British army does.  | 
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