congrats in your new family member! I love shelites :)
You are very correct to worry about ivermectin toxicy in collies. It's not just ivermectin but a whole host of meds. You can actually test for the gene of it would make you feel better.That being said the dose of ivermectin in heartgaurd is really low and very very safe even for ivermectin sensitive dogs.
Revolution is a good combo product but I think there are better products out there for heartworm prevention. Advantage multi is another topical combo product, pretty similar to revolution but I trust it more for hw. Trifexis is also a combo product that does both flea and hw but its oral so there is no topical residue. If I didn't live in the heart of tick country I would have my dog on Trifexis no doubt about it.