[Vanilla slimes]

I've had an issue with that,actually.Could never use a blue slimeball to make the Xeno's Reliquary handgun,tried on single and multiplayer,and forge's ore dictionary cleary has an issue with it.Don't recall many other issues,though.

[Mob heads?]

I've seen vanilla mobs drop heads before,and there are skeleton heads all around in caves,though maybe just in ice biomes.(Never confirmed that)

[Endermen]

I'm going to be stupid here and ask,are you doing this at night?And what weapons do you use,for that matter?I've had them disappear almost completely with any weapons that have a fire enchantment.