My girls take Heartguard Plus for under 25 pounds and have never had a problem in any respect, thank goodness.
For fleas and ticks we use Frontline. It is also based on size, and you just squirt it between the shoulder blades and it protects them for a month. We haven't had a problem with that either.
I recommend, according to my Vet, that you do not give them both medications at the same day - try to space them out a little bit.
Have fun on your trip!
