algunos and unos are mostly interchangeable. If you are interested in understanding the subtle differences between their uses then here is a helpful link http://spanish.about.com/od/translationsfromenglish/a/algunos-unos.htm
To add to the other commenters, I would like to point out that there is a discernible distinction between "expert" and "specialist". An expert is someone with expertise in a given field or activity. A specialist is someone who specializes in a given field or activity. Although both are very similar, the distinction lies in the level of exclusivity and dedication: an expert is anyone with expertise in something (when speaking about said thing, often in the form of "expert [in/at/of] X"), whereas a specialist is someone who specifically concentrates their efforts on that thing, typically a subject or activity and often contrasted with "generalist". Seeing the definitions for each term can elucidate on their distinct meanings. Here are the first definitions for each term:
An expert is "[a] person who is very knowledgeable about or skilful in a particular area".
A specialist is "[a] person who concentrates primarily on a particular subject or activity; a person highly skilled in a specific and restricted field".
The terms are definitely synonyms, but to translate "especialistas" as "experts" when "specialists" is more apt is unnecessary, especially when the term "experto" already exists for "expert".
No! These off beat words should not be accepted. Duolingo is trying to keep things simple and our learning as easily as can be done. We are not studying to be professional translators, here. Might as well be bemoaning about how, authority, buff connoisseur, consultant, expert, hotshot, master, maven, professional, whizz , also do not work.
One can clearly see that especialista translates to specialist, so why not use ithat word? What is the point to trying to use words out side of what Duo is teaching. WHY? What is the point of doing this off beat business and then crying in Comments about some off beat word did not work? I really would like to understand this.
This is rather subtle. Some or A few (emphasising the 'a') are equivalent, a relatively small number, not "many" But if one says "few..." you are making a point that the number is too small, insufficient for purpose or not as many as you expected. You never use the indefinite article with some by the way.
Please correct me if I am wrong, but I think that was specific to "pregunta". DL has plenty of other examples of algunos/as. In a reversed way in English we almost always use the plural: "Any questions? Any answers?" Although I believe the singular is grammatically correct for both, they are rarely used.
Maybe the logic is similar to this from another discussion: Mi habitación no tiene ninguna ventana. Why is ventana used and not ventanas? Spanish does this sort of thing. If the room not does not have one window, why say it does not have two? If three of us eat an apple each, Spanish say we eat an apple, not three. If a classroom of students raise their hands, Spanish says hand, unless we intend to say two hands per student.
Yes it would be wrong, "algún" is used when the noun is singular e.g. algún estudiante.
When the noun is plural it changes to "algunos" e.g. algunos especialistas
Not all words that end in "a" will be feminine. Another example of this is "deportista". Nouns like this may refer to both males and females (you could say both "el deportista" and "la deportista"). If the gender is unknown or if there is at least one male in the group then the masculine form is used.