>I would be quite interested in finding out if there are any fossil remains of mammals and how they fit into the ecosystem with dinosaurs before the big one hit
There are, plenty. The oldest mammal fossils are between 150 and 200 millions years old. Mammals and Dinosaurs coexisted for a very long time. We identify early mammals by their teeth. Mammals alone have precisely interlocking teeth. This came at a price. Sharks and crocodiles can replace lost teeth indefinitely - but when you have precisely interlocking teeth every tooth is a snowflake, and so you can't just sausage-factory out infinite replacements. Mammals therefore only have two sets of teeth - one smaller set that sees them through childhood and a larger set through adulthood. All our dental issues and root cannals began with that.
But it has a catch - to make the first set last through childhood, it had to be bigger than what can fit infancy - so for the first part of their lives mammal babies have no teeth at all. So they needed a new food source for babies. Thus was evolved: milk.
So the teeth are a key clue to whether or not a creature was milk-producing, and it's how we differentiate early mammals from their reptilian contemporaries and close ancestors. The reason the date-span is so long (150-200 million) is that the oldest likely mammal fossil we have is 200-million years old, but many paleontologists believe it should be considered a reptile ancestor of mammals and not a true mammal yet. By 150-milliion years ago though, there were plenty of mammals and they were definitely mammals. These first definite mammals were morganucodontids which were tiny creatures that looked rather like shrews. They probably at seeds and the occasional dinosaur egg and were likely eaten by the smaller predatory dinosaurs in turn. They were however, brainy little guys. Their skull cavity for body mass ratio was far higher than any known dinosaur. They were our ancestors - and the mammalian trait of intelligence was already established.
By the time of the K/T event they had diversified significantly into a number of species. What the study now actually says is that most of those species did not survive K/T - only a small number here and there made it through. And then, as plantlife recovered, there were these massive ecosystem niches ready to be taken advantage off - and no big creatures in the way, and those mammals were perfectly poised to take advantage. You often find the greatest diversity right after mass extinctions. With so many creatures gone, for a while almost any body plan can offer a workable survival advantage - and then as they start to compet with each other, it narrows down again into the winning categories.