1. Mines is not as good as yours. (Is 'yours' singular here? Or can it refer to more than one person?)
2. Mines is not as good as you guys'. (Can I use 'yours' as in 'you guys' '? Like refer to more than oner person.)
First "Mines is " is not correct. Though close. Since your talking about multiple things that you have "Mine are" Would be correct. Or else you are either talking about someone who is named "Mine" in which case that would be fine as long as you used it like "Mine's". But if your talking about a singular thing Just "Mine is" would be perfect.
Are is used for multiple things, so it takes the place of the "s" at the end of Mine, and the "is" Because that would just show your talking about one thing.
Also useing "You guys" is incorrect. Using "You guys" in that sentence makes it sound like your talking about the people and not the people's possession.
That may be a little confusing, but if there is something that I said that you don't understand, just ask. :)
But no, using "Yours" is not only singular. Its talking in the general direction. :)