"Good" familiars are just more rare and expensive. There is not anything inherently different about how they work in the game, except that you can only have one of each kind of familiar in your lair - if one of your dragons has a Luna Mith, you have no use for a second Luna Mith because a second dragon can't take it. I always go for the cheapest familiar I can find, in the auction house or the marketplace, when I have more dragons than familiars. (Sometimes I have more familiars than dragons, and then I end up swapping out a bunch of familiars onto one dragon to bond with them all - you can do that and it won't lose you anything.)

"collect everything that's free; you'll use it all eventually; don't pay for anything until you figure out whether you have a use for it?" is definitely my strategy when it comes to Stuff. I especially like having as much of as much stuff as possible because Crim the Collector asks for random objects at twice the price you'll get for using the 'sell' option in your hoard, and the more random objects I've got, the likelier I will have whatever she wants at any given time. (Sometimes she asks way less than something is actually worth - I usually don't sell battle stones to her, and you definitely shouldn't sell scrolls to her; it is almost never worth it.)

I love love love opening treasure chests, so I open all the ones I get. Other people might prefer to collect and/or sell them. Selling them might be more profitable, but to me at least, opening them is much more fun.

There is not a limit on how big a hoard can get, as far as I know.

I am happy to help! :D