Can I translate "Μερικοί σκύλοι" as "several dogs"?
Not really. "Μερικοί" just tells us there were dogs. "Several" show that there were many dogs.
Is the "now" necessary when there is the gerund "are drinking" in that sentence?
Τώρα means 'now'.
"Now" can be inserted in more than one place, including after "are"
Good catch. We've added "Now some dogs..." and "Some dogs are now drinking..." Thank you.
A few is ok yes