I recall something from Zamenhof that I read years ago saying that the English R was an acceptable substitution. I can't find it now, but it basically said that since every European language has its own "R-sound" and they're all mutually recognizable as an "R-sound," you can pronounce the Esperanto R as you would in your native language. He is said to have recommended Italian as a model for Esperanto pronunciation—which is apparently what they're going for on Duolingo—but as a proposed international language, it was never going to have a standard phonology no matter how he tried.
If you check out http://en.wikipedia.org/wiki/Esperanto_phonology#Assimilation the list of accepted allophones is huge. One of the main criticisms of Esperanto is/was that it's too rigid to the point that it becomes uncomfortable to speak. This "L+R" is just one example of that. In reality, difficult sounds are often merged or "assimilated" with the sounds around them. In Spanish, for example, the L+R combination is sometimes reduced in rapid speech such that only the R is fully pronounced (becoming RR in the process), while the L sort of merges with the vowel before it or receives an extremely weak pronunciation (this is hard to explain without getting too technical, but the Spanish Phonology Wikipedia page gives the example "alrededor.").
Zamenhof apparently didn't want this to happen to Esperanto. In my opinion, he was nuts for thinking he could prevent it.
It doesn't really matter in English, either, so that should only make it easier to remember. You could say "they sing slowly," but "they slowly sing" is also perfectly acceptable, if somewhat awkward. Now, that applies to verbs. Note that just now, I used "perfectly" to modify the adjective "acceptable," and I certainly wouldn't recommend putting an adverb after an adjective that it modifies. Not in English, anyway. I'm still new to Esperanto, so I'll find out soon enough I guess.
Because the verb in use is kantas (singing). And "malrapide" is an adverb (ends in "e") describing how they are singing. If you wanted the adjective, one would change the "-e" suffix to an "-a" to make "malrapida."
My translation is likely crap, but your example sentence, "They are slow singers." would translate to something like, "Ili estas malrapida kantistoj."