Hi and welcome to the forum.
Labs and chihuahuas are very different dogs--not just in size. Just remember they ARE dogs, not tiny little humans! Most chihuahuas have a chip on their shoulders and you have to be careful with interactions with other dogs. I assume that Blue Cross is a shelter? They should be able to tell you about their temperments. Have some beds around that they can fit into together. Get some samples of whatever the shelter feeds, or at least buy a small amount. Bad enough to go to a new home, but with different foods, you might end up with 2 dogs with diarrhea. I use a pen for my chi's. I use adult human pee pads in the pen, so if they can't last the night, I don't have to worry. I open the pens during the day. I use the 'iris' brand of pen. Get a few toys for them. Get some things to chew on---the "Kong" has great samples to chew on. Will they be chipped? If not have that done soon. Leash and harness's too. Get down on your knees, if you are able, and see if there are loose cords etc that the dogs would like to chew on! You can buy some stuff to put on cords etc that attract the dogs that they shouldn't have. I don't know how old these two are or why they ended up at the shelter. Good luck, and we are here to answer any questions you have.