is wearing a longer leash safer even if you have a small board?
Pretty much any leash is fine for a shortboard in small waves a 6ft leash is ideal. You'll want a bigger leash if you start surfing waves that are bigger than you are but you won't be doing that for a while. The longer the leash the more it will drag in the water behind you so it is not practical in small surf.
is double swivel realy useful?
I think that they are good and help protect against the leash twisting up. I would go for one every time.
Any leash you buy from a surf shop will be fine fo small waves - they are pretty strong and should last you for years of small wave surfing.