In LoC Rand specifically states tht there are 2 kinds of Darkhounds; ones that can be killed normally, and ones that can't. We have seen evidence of this as well. When the Darkhounds first appeared in EotW, some were taken out with arrows (first time we see the Two Rivers Longbow in action), while in Ruiden(sp) Rand had to use balefire.

The theory that the Darkhounds Rand had to balefire were somehow imbued with the Dark One's essance because he was specifically looking for Asmo is flawed. They found Asmo first but left him alone. They snuck past everyone in the Maiden's house to get to Rand so in that we can assume they were following orders. The same with the one that went after Matt. Logically, Matt and Rand were the targets of the attack.

The argument that it is just Rand's opinion that there are 2 types of Darkhounds is further damaged when you consider that it is during this time that we start to see instances of Lews Therin's factual knowledge begin to leak through into Rand's conversations with other characters.

It is my opinion that there are 2 types, and that Rand was speaking with Lews Therin's knowledge on the subject when he made the statement.