I'll try to avoid getting on a soapbox too much - but I personally would not feed a cat Science Diet. It's not grain-free. It's full of undigestible crap like rice flour, wheat flour, corn starch, and cellulose. Cats do not need ANY grains in their diets. They are 100% carnivores. While dogs can benefit from SOME grain or grain by-products in their food, cats do not thrive on grain in any form.
I spent a long time looking around for different grain-free cat foods, and I'll be honest, they're usually more expensive than most cat foods you find in the pet section of the grocery store. But there are many good grain-free foods out there. I have fed my cat Weruva, Wellness, Merrick Before Grain, and Ziwipeak. She likes Weruva the best.
In short, a cat probably won't die from eating food with grains or flour in them, but it's just not really nutritionally appropriate. You aren't doing anything bad to your new kitty by feeding him Science Diet, but I think he would have a much longer, healthier, and happier life on a grain-free diet.
Also, wet/canned food is definitely the way to go. No dry kibble ;}
When you take Smokey to be neutered, see if you can get the vet to check out his hind legs. He's way too young for arthritis or joint issues, but he may have an old or new injury, or even a slight deformity, that gives him pain or discomfort in his hind legs. The vet will probably rotate the legs to check out Smokey's range of motion. It's also possible that he isn't injured, but just literally hates having his hind legs touched. My cat doesn't mind having her front paws or ears or even her tail touched, but if you so much as get within one inch of touching her hind paws, she'll move away. She doesn't have any pain in them, she just refuses to let me touch them, ever.
Here are some good, well-layed-out "plans" for introducing a new kitty to his animal family:
4PAWS - Introducing A New Cat
Introducing a New Cat to Other Pets : The Humane Society of the United States
Good luck! :}