Yeah, they sound about right. I think I might be able to take the Black Knight since I took out the one in the church
(are minor spoilers allowed in this thread btw? Wanted to be sure

)
Yeah, you are fine, but you can always tag if you don't want to take any chances.
My advice for each:
-Black Knight: lure him out to an open space. Trying to fight him in narrow space is suicide at an early level. Keep your guard up and constantly be moving around him. He will keep pace with you most of the time, but he'll eventually try striking, opening himself up for backstabs. If you haven't gotten backstabs down (assuming you aren't a sorcerer class), practice a lot against the dipshit undead that are all over the Undead Burg. Unless you are comfortable with it, I wouldn't even attempt trying to parry him. If you mess up you will be punished dearly for it against him.
-Havel: same philosophy, but fight him where you find him. His club is
devastating, but he's slow and opens himself up a lot to the backstab. It will take some time, but with patience you can get him. The good thing with Havel is that he's right by the Taurus boss (Havel is down the staircase, Taurus is up). If you get some summon help, you can generally take them with you to fight Havel (as he's not considered a boss and the summoned will not disappear), then go right up for the real boss fight.
-Gargoyle: don't even mess around. Reverse hollowing by the blacksmith, go outside of the church, and summon a phantom. Unless you are fairly powerful, you will get 3/4 through the Gargoyle's health when (just in case you want to go in fresh)
Having back-up will make this battle simple. Just keep that guard up.
-Headless Demon: if you are coming from the blacksmith side, dodge-roll his bolts, run past him, and get to the stairs. If you can keep the demon at the top of the stairs you can get a slight advantage when we whips his trident around, as you will be on a lower level. Learn his patterns, go in for a few hacks, move away, but not too far or he'll blast lightning at you. Two-hand your best weapon (if you haven't gotten the Drake Sword already I highly recommend looking it up. Even if you can't one-hand wield it, it's two-hand heavy hit is really powerful) and chip away.
Hope some of this helps. The key to all of them is just patience, keeping your guard up, and constant movement.