Here is an answer I always give as far as the food issue goes...
You have a large breed pup so don't feed high protein/grain free puppy food until the dog is over a year old or it can cause rapid growth and bone problems. After a year grean free/high protein foods are ideal.
Have you considered feeding a raw diet? How bout a home cooked diet? In my opinion, these are your best options. If this is too inconvenient, I'd suggest choosing a high quality kibble. Most of the foods you see on television commercials and sold at the grocery stores aren't the best choices. Look for a high quality, natural food without artificial preservatives, colors, additives, fillers, corn, wheat, or soy as a initial rule of thumb. Grain free is the best way to go, which generally has high amounts of meat protein. If price is an issue, don't sweat it. There are plenty of good foods out there that are priced the same as poor quality foods. Here is a great site to learn more about what foods are the best and the worst, and their pros and cons. The 6 star foods are the best and the 1 star foods are the worse. I'd at least try to feed 4 star foods, but of course 6 star foods would be ideal. Here are some brands I'd recommend off the top of my head;
Best (Don't use these foods until your dog is an adult/done growing. At least until one year or more, maybe even less, depending on breed. These foods have too high amounts of overall nutrition and cause larger breed dogs to grow too fast and develop bone problems. Wait until adult)
EVO
Taste Of The Wild
Origin
Timberwolf Organics
Canidae Grain-Free Formula
Wellness CORE
Very Good
Innova
Wellness
Canidae
California Natural (Great For Dogs With Food Allergies)
Solid Gold
Good For Best Quality On A Budget
Fromm
Premium Edge
Natural Balance
Diamond Naturals
*Don't forget to consider home cooked or especially raw (which is EXTREMELY cost effective). Let me know if these interest you and I can give you more info. Good Luck!
Great resource for kibble reviews:
Dog Food Analysis - Reviews of kibble
*BTW... Blue Seal Dog Food is not a very good food. Pretty much on par with Iams, Purina, etc which aren't very good foods in my opinion. I'd slowly switch over.
As far as the cat goes it depends of what type of breed you end up getting and how prey driven the dog is. when my dog was a pup she was curious of the cat but a few swats later and the dog kept its distance

Sometimes they figure things out themselves. Just see what happens at first. If the dog chases or things get out of hand have them first meet through a glass window or baby gate so they can see/sniff one another but not get at one another. Then if the dog still goes after the cat you have to work with methods such as removing the dog from the situation and saying no firly or distraction methods with treats/toys. If you need to keep the dog on lead do so. you may never have an issue though so if you do update us and I'd be happy to give further advice.
As far as the leash goes you need to obviously teach the dog to accept the leash. keep it on him for a few hours and let him just drag a loose one around to get used to it. After, try walkig a bit even just around the house. use a treat or toy as a guide if you have to. use positive reinforcement such as treats and an encouraging voice to make the dog happy. If he bites at the leash or pull simply stop in your tracks and firmly say "no" or "leave it". If the dog complies praise him/treat him. you could use a clicker to go along with the treat if you'd like. This is often a good strategy to get attention while distracted outside which can lead to pulling or stopping in which case you could click it and the dog would come for the treat and continue the walk. You want to make the dog WANT to do what you're asking and make the dog feel he or she is getting something positive out if cooperating with you. That;s where the whole positive reinforcement thing comes into play.
Good luck!