Vet visit went well. Nim was surprisingly calm throughout, and they did prepare a nexgard solution. I'm not 100% confident in their weighing of him, so I'll double check that before treating (it came in lower than I expected at 0.28 lbs, but I haven't actually weighed Nim myself yet.) At least I know it's probably not an overestimate for the dosage. I fed Nim last friday, so I'll give him the nexgard rat in a few days.
I'd feel more confident overall if I had prepared the nexgard myself, but definitely less of a hassle to get the stuff this way. The vet also gave me ointment for his scar she thought might have been from a rat (which he arrived with, and which I suspect is a burn.) It seems already healed so I don't think that'll do anything, but I'll use it just in case. The vet said he looks healthy overall, but suggested I bring a stool sample in case the mites are indicative of other husbandry issues (worms.) I have no confidence in Nim's seller, so I'll probably do that.