The following system in the game is currently very limited; it can only handle a certain number of independent creatures following you at once, and more than that simply get left behind, even when they are linked and chained properly. The easiest way to see this in action is to pick a class that can learn find familiar, then have each of your companions learn it as well - most can if you pick the right subclass. When everyone has a familiar out, no more than five creatures/characters will follow; the rest will just stand still and be left behind.
That's not true. I've had no problem with seven characters in a party. Four players, a pet, a familiar, and Halsin. The only current issue I came across was that the spider pet won't/can't use ladders. He can't go up at all. And if he's already up, and the party goes down, he ends up jumping and taking damage. I haven't actually tried with other pets, so I'm not sure if they have problems with ladders. Imps don't have the problem because they can fly.
There was an issue early on, where other characters wouldn't jump over gaps or barriers to follow you, but that has been fixed. And Astarion won't follow across water, but there is a reason for that. And even in that case, you can take control of him and make him cross and take the damage.