Your questions are a bit vague, and also, I don't think валютный курс is a phrase that's likely to be used outside of an academic or professional discussion that's more about the notion of currency exchange rate itself than the rate of a particular currency of a particular point.
But to try to answer your questions anyway:
Can we form the 1st sentence like the 2nd sentence and the 2nd sentence like the 1st one?
Yes and it depends, respectively. Курс валюты is fairly common, курсы валют even more so, though again it's more likely to refer to exchange rates in general or in theory.
Here, I was about to say that unlike with the first phrase, долларовый курс is not the same as курс доллара, but then I checked with Google and was surprised. Apparently долларовый курс is seeing some use. This is new and weird to me. It's certainly something I'd never say (unless perhaps I was talking about the rate of some other currency expressed in dollars, but the examples I've found are clearly talking about USD/RUB). I'd still maintain курс доллара is the better phrase to use, I'm yet to hear anyone use долларовый курс in everyday spoken language, and I'm tempted to go into disgruntled purist mode and say that it's probably more of that strained artificial journalese that's been infesting everything; but it's probably wiser to seek a second opinion on this.
When do we use the adjective phrase and the noun phrase in genitive form?
I'm afraid there are simply no rules, only habits. Russian doesn't have something like the Germanic compound or the Semitic idafah; out of interest, I once took an English noun ("fox") and found at least thirteen different, mostly non-interchangeable ways a simple compound — "fox" + another noun, as in "fox hunt" or "fox fur" — can be expressed in Russian. (Most are noun phrases involving various oblique cases and sometimes prepositions, but compounding also exists in Russian, to a limited degree compared to English, which however makes it none the less mandatory where it just happens to be mandatory, as dictated by the established usage or precedent. "Fox hater" would require the occasionalism лисоненавистник.)
In some cases, it's not an issue: автобусная остановка and остановка автобуса are fairly interchangeable; but these are the exception rather than the rule. If there's any rule, it's that adjective phrases tend to be more fixed and stable: сахарная вата "cotton candy" vs. the ever-so-slightly more ad hoc уровень сахара "sugar level" (вата сахара or сахарный уровень would at best sound like you're being deliberately silly), but then again, the more important fact here is that уровень just happens to be a noun that doesn't "like" adjective phrases where it could do with a noun one.
On the other hand, a noun like шахта is very noun-phrase-phobic, with a single notable exception that proves the rule. "Coal mine", "gold mine", "uranium mine" would be угольная шахта, золотая шахта and урановая шахта; шахта угля, etc. would be weird without even being funny. And the exception is шахта лифта "elevator shaft"; again, лифтовая шахта isn't something one would even think of saying. Similar to the whole ordeal of Russian stressed syllables, there are so many implicit rules here that it's actually simpler to say there are no rules, and you have to find out and follow established usage to be able to decide whether it will be an adjective phrase, a noun phrase, or the occasional compound. Just like, I'm afraid, with the famous and dreaded Chinese counting words.