|1) WHY IS HE MEAN TO HER
How did you bring her into the house? Was he introduced to her somewhere other than 'his territory'? Does she get more attention than he does?
Chihuahuas bond very tightly to 'their human' and they don't like to share. If you just brought Tequila home and installed her in 'his house', he's not going to like it one bit. He may be feeling sibling rivalry. I know it's tempting to give a lot of attention to the new puppy, but You MUST make sure that you give at least as much if not more attention to Taco as you do to Tequila.
Here's what I did, and you may want to try this. I took Gremlin (3 pounds and just over a year old) with me when I went on the search for a puppy. She got to meet puppies at two different breeders, and she actually chose the puppy she wanted (luckily it was also the puppy I wanted). We took the new puppy home and Gremlin accepted her (she thought it was Her idea to get her a playmate). She was quite tolerant of Nixie's puppy playfulness and took it upon herself to potty train her little sister.
Nixie is now almost a year old, and outside of the normal juggling for position there's no problem. Gremlin is still the top dog, even though Nixie weighs almost double what she does. Gremlin has her own litter tray and her own food dish.
Now, I know you can't go puppy searching, but have you considered taking them both to somewhere that is not your house? Go visit a friend and see if Taco is less nasty to Tequila. Be aware that there may be a lot of snarling snapping wrestling. This is normal, it is how they establish their place in the pack, as long as there is no loud yelping or blood, observe but do not interfere. Unless there is obvious damage being done, do not wade in, pick up Tequila and comfort her while yelling at Taco. This just makes it worse, because now he has to assert his dominance over her even more. However hard it is, do your best to ignore them both, let them 'fight it out'.
Assuming he treats her reasonably well, now you know it is a 'territory' issue. Bring them back home and lavish your affection on Taco "Good dog, good Taco, share your house with Tequila" while ignoring Tequila. This reassures him and shows him that he is top dog, which should reduce his need to assert it over Tequila. Make sure that he gets fed first and that he gets just that little bit more attention than Tequila. He may be allowed on your lap, Tequila has to settle for being on the sofa next to you, that sort of thing. You have two hands and two chihuahuas, use one to pet each of them at the same time, another way to reassure Taco that he is not being bumped from his place in the pack.
If you still have aggression or obvious damage being done to Tequila, you need to work with a trainer or consider finding Tequila another home, then start again using the method I outlined above to introduce a new puppy into 'his house'. I'm also going to make my plug for minimally having Tequila spayed, it's much more humane than leaving her to go through heat and much easier for you to not have to worry about keeping them apart. Most female chihuahuas are not good candidates for motherhood, even experienced breeders often lose the mother and/or the litter.
She's just fine, odds are her ears just haven't gotten the knack yet. You can help by massaging her ears. Rub the sides of her face up and down so her ears go with, and from front to back. You are doing it too hard if her eyebrows end up at the top of her head, or if she looks like she's had a bad face lift. This will encourage the muscles to get stronger.
If they do perk up, one might go before the other one, keep doing the same massage to both. One or both might droop again when she starts losing her baby teeth (around 4 months if I remember correctly, my Nixie did that, just the right ear). My advice is take a few pictures, 'cause they're So Cute like this. They should perk back up, continue the massage, which is also a great way to give attention and affection, chihuahuas just love to be touched.
If by the time she's six months old they aren't standing up by themselves, then they probably never will. She's still a chihuahua, she doesn't care about her ears and you shouldn't either. You can always tell people who ask "Tequila is very rare, she's a floppy eared chihuahua". Most people will be impressed, and those of us who know better will just wink and smile.
This is the advice of one chihuahua owner, and I don't know the whole situation, so I always recommend that you listen to your heart. You obviously want the best for both your dogs, that makes you special and them lucky. I also recommend the book Chihuahuas for Dummies, which is the best book I have on the care and understanding of these adorable little creatures.