If you stick to the left it's easy to avoid them. You'll have to run away from one, and around another, but you should get past them with no problem.
Might want to make sure you have a Homeward Bone handy. It can be a bit tough getting past them on the way back.
Yeah, you need to be in human form, and just activate any summoning signs. It might be tough to summon people to fight Sif though, because of invading Forest Hunters, and a lot of people place their summoning signs down to help people troll Forest Hunters. I'd suggest playing offline until you get to the bridge leading to Sif, then go online and see if there's any summoning signs on the ground.
On my first playthrough, I used a Titanite Catch Pole for a while, which was nice because it's very powerful and has nice range, but it's also a rare drop from Titanite Demons.
I also used a Silver Knight Sword from the knights in Anor Londo, which was my most used weapon for playthrough 1 until I could finally make Quelaag's Furysword.
Uchigatana, Iaito, and Fishing Pole are all good for dexterity builds if that's what you're going for.
And yes, you usually need to heal people you summon. They can heal themselves with a Miracle or Humanity, but usually won't. I'm pretty sure Phantoms only receive a fraction of the healing from an Estus Flask as the host does, so keep that in mind.