What we Know:
"the Guardians' anatomy and physiology, you might be surprised to learn they weren't that different to us. Their morphology was bipedal, although they were generally taller and more slender than the average human. Their skin tone was red, encompassing the whole spectrum of red hues. There were various reasons for these changes in pigmentation, including genetic disposition and the fact that they darkened as they aged."
"Their faces were proportionally smaller than ours and they had round eyes, darkened from their evolution under a very strong sunlight. The colouration and structure of the eye indicates that not only they have keener vision than us, but they could also see a wider spectrum."
"The nose appears to have been vestigial - I believe it was once more pronounced, but that some environmental change made it less useful - so their sense of smell would have been less developed than the human average. Their ears were very tightly formed against the skull, and the interesting array of internal acoustic chambers suggest they perceived sounds differently to us."
"Most remarkable are their limbs. Their forelimbs had a serrated edge and were no doubt used as killing implements during the pack-hunting period. They had two more more joints in their arms than we do, facilitating complex articulation and the use of sophisticated sign language. The skeleton was flexible yet resilient and their muscles were long, thin and ropey. They had two hands, each with four digits."
and "not only were their arm ridges excellent for killing prey, they also used for cutting meat into little pieces to fit in their small mouths."
So, tall, thin, dark, round eyes, various shades of red, ears right up against their heads, with serrated-edged forearms and small mouths - I'm seeing this to be something akin to a preying mantis:
I'm picturing something more along the lines of Mass Effect's Salarians.
We don't have any mention of hair, so they may or may not have had any hair. We do know they preferred warmer climates, and were less tolerant of cooler ones, which suggest a lack of hair.
I don't find "space elves" to be a very apt description at all.
"the Guardians' anatomy and physiology, you might be surprised to learn they weren't that different to us. Their morphology was bipedal, although they were generally taller and more slender than the average human. Their skin tone was red, encompassing the whole spectrum of red hues. There were various reasons for these changes in pigmentation, including genetic disposition and the fact that they darkened as they aged."
"Their faces were proportionally smaller than ours and they had round eyes, darkened from their evolution under a very strong sunlight. The colouration and structure of the eye indicates that not only they have keener vision than us, but they could also see a wider spectrum."
"The nose appears to have been vestigial - I believe it was once more pronounced, but that some environmental change made it less useful - so their sense of smell would have been less developed than the human average. Their ears were very tightly formed against the skull, and the interesting array of internal acoustic chambers suggest they perceived sounds differently to us."
"Most remarkable are their limbs. Their forelimbs had a serrated edge and were no doubt used as killing implements during the pack-hunting period. They had two more more joints in their arms than we do, facilitating complex articulation and the use of sophisticated sign language. The skeleton was flexible yet resilient and their muscles were long, thin and ropey. They had two hands, each with four digits."
and "not only were their arm ridges excellent for killing prey, they also used for cutting meat into little pieces to fit in their small mouths."
So, tall, thin, dark, round eyes, various shades of red, ears right up against their heads, with serrated-edged forearms and small mouths - I'm seeing this to be something akin to a preying mantis:

I'm picturing something more along the lines of Mass Effect's Salarians.

We don't have any mention of hair, so they may or may not have had any hair. We do know they preferred warmer climates, and were less tolerant of cooler ones, which suggest a lack of hair.
I don't find "space elves" to be a very apt description at all.