Does anyone have any negatives about Maltese/Shih Tzu's?

Too hyper, too affectionate, hard to train, pee too much.
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Yes — there is no such thnig as a "Malshis" - it's a MUTT! Most likely it will be "bred" by a disreputable BYB who is selling "designer dogs" to make a few bucks.

Try going to the shelter and rescuing a nice mixed breed (aka mutt). And please, when you get it home, remember it's a DOG and not a DOLL.

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They're mutts bred for money, not health. If you want a mix, go to a rescue. Otherwise, find a responsible breeder for a purebred.
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I have a Shih Tzu and its awesome. Ive heard about Maltese/Shih Tzus and they are very affectionate and cute. I think you should keep Lucas or if you really want to get a new one
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if you do get a new dog, don't forget about poor lucas. "just be happy with lucas?" it sounds as though you don't want him. Like you said, Lucas is six. I bet he's very attatched to you. But if you can still give your love to Lucas as well as your new dog, I don't see why not.
And I'm not sure about your negative question. I'm not very forte in the small dog region. :]
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"Malshi's" are a MUTT.. there's no guarantees on size, color, coat type, health, personality, temperment, etc. You could end up with a dog that is the same size or larger than Lucas with Shih Tzu markings.

Dogs are also not meant to be dressed up. Not all dogs enjoy it. If you want something to play dress up with, then get a barbie doll.
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getting a new dog would be a great idea !! maltese are very affectionate animals,and love other small dogs,however,i have a shih tzu,and when she was a puppy ,she got into,and chewed up everything !! from my shoes,to my shirts,so keep everything up ,until her adult teeth grow in,and buy lots!! of chew toys.
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Poor Lucas! He is going to be pushed out of the picture, so to speak, if you get another dog. First you'll be spending all your time with the new puppy and then you'll be dressing her up all the time. After six years of love and devotion, this is the reward he gets? There will be problems, too. Dogs are territorial and Lucas won't like his territory being invaded by a small white stranger who is stealing all the attention. Lucas will begin to pee in the house and maybe poo, too. It will be natural for him to growl and snap at the puppy and he'll probably get punished for that, too, while he was only trying to defend his territory.

In nature, females are always alpha so Lucas will not only lose his territory but he will also lose his position as alpha dog. The smaller dog is going to steal his bed, his toys, his treats, and most of his time with you.

IMO, Lucas is not the only one too old to play dress-up. You are, too. Dress-up games are for little girls with dollies. What happens to the new dog when you get tired of playing dress-up with her? And what happens when you go away to college in a couple of years? Do your parents want to keep TWO dogs? Maybe two dogs that don't like each other much? If they have to pick only one, who gets the boot, poor old Lucas?

Also IMO, you should just be happy with Lucas. If you aren't already, you'll be dating soon and hanging out with your friends or doing homework the rest of the time and will have hardly any time left over for even a goldfish.

I know this hasn't been pleasant to read but it is factual.
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Please support rescue. NEVER, EVER give a cent to people who are breeding mixed breed dogs. I am not being rude, I am telling you the truth. Regardless of their claims, they are ALL backyard breeders and puppymills, as all they care about is money. They don't care about the thousands of dogs, purebred and mixed, young and old, that are literally dying as they wait for a new home. They also don't care about the health and temperament of the animals that they breed. By buying from these "breeders" you are supporting the puppy mill industry.

http://www.prisonersofgreed.org/
http://www.awarenessday.org/

Further information about designer mutts:

http://www.shihtzu.org/Info/designer_dogs.asp?menu=Info
http://www.shihtzu.org/Info/more_on_designer_dogs.asp?menu=Info

ANY dog/cat/rabbit/etc of any breed, mix or age can be found in shelters and breed rescues, as this video will tell you:

http://www.youtube.com/watch?v=emGZBLVJmGI

Shelters and rescues, both all-breed and specific-breed rescue, are filled to the brim with designer mutts, and here are just a few of them:

Your local animal shelters
http://www.poomixrescue.com has many dogs marked URGENT because they are in danger of being euthanized
http://www.petfinder.org/
http://www.dogsindanger.com/
http://shihtzu.org/ASTC/rescue.asp?menu=ASTC
http://www.americanmaltese.org/ama_rescue.htm

~*~If you're interested in a dog but live too far away from it, there is the option of having a dog transported to you/near you. Google "dog rescue transportation."~*~
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10+ years of being a shelter volunteer, pet owner and lifelong animal lover