We train puppies to the stove pellets starting at 3 1/2 weeks old, but they are hardwood, not pine. I don't see why they wouldn't work though. They break down from the urine into saw dust and I've never seen any that contain walnut. You can buy them from manufacturers for 5 bucks a 40 lb. bag in pallets of 50 or something like 6 or 7 bucks each from Tractor Supply. There is a manufacturer in Louisburg, N.C. but you can find them all over the country with a Google search if you want to buy in quantity. The manufacturer can tell you the varieties of hardwood in them, but the ones I've seen are mostly Poplar, Maple, and Sweet Gum.
We've used the equine pine pellets in a pinch for the puppies when we ran out and couldn't get the hardwood pellets. The pine always smells stronger, but I don't see any difference in how they work.
It all depends upon the brand of pellets. The woodstove pellets we use state on the bag that they are 100% pine or fir with no accellerants or glue, as do almost all of the pellets sold around here (NW Oregon, land of the Douglas Fir).
Just check the bag -- those that don't disclose the contents aren't even considered. I won't use WalMart pellets for that reason, but I'm very happy with Beaver woodstove pellets and those by Blazer.