I just got home from my vet with my Charlie, He went in for his 4th vaccination series shot. He is 16 weeks old. He got a lepto this time. My vet does not give the lepto until they are older. I have never had any reaction to the lepto when given to pups 16 weeks or older. I have been going to my vet for 30+ years. He does not recommend bypassing the lepto, just wait until the pup is older.
Heres a good site on Lepto.
Leptospirosis in Dogs