I would not use the pour-on. Injectable can be used orally for dogs, but vets do not reccomend it, because it is quite easy to cause permanent kidney and liver damage with a poorly measured dose. I use the 1% injectable solution orally for my dogs, but I started doing it when I was working as a vet tech, and I did it under a vet's direct instruction. So, if you trust your own measuring skills, the dosage is 1/10 mL of 1% solution per 10lbs of body weight. This is still a much higher dose than what you find in Heartgard.
This is the dose used for filarial clearing, rather than the dose used for monthly prevention. However, this will do no harm and it provides the same protection.
The first time you do it, do it on a weekday during business hours, in case your dog has a reaction. It is unlikely, but I'm an over-planner, it's what I would do
Davidb, the reason Collies are sensitive is because of a mutation in the gene that codes for P-glycoprotein, which functions in regulating the blood-brain barrier. Because of this mutation, their bodies cannot process the drug properly, and it accumulates in brain tissue. This causes horrendous neurological damage.
I don't know what to say about your statement that dogs all have the same genetic makeup. Genetic variation and selective breeding is the *source* of all the varieties of domestic animals.