You can use the trichlor but just monitor your CYA so you can quit using them when you get CYA to 40-50. But, it will take a bit before they get some CYA in there so for a few days, you'll probably need to keep an eye to make sure you have chlorine in there. Or, your other option is also ok, use a little CYA to get to maybe 20 or so and then use the trichlor to get the rest of the way to 40-50. Any way will work. Just have to monitor things. Also remember that trichlor is very acidic so it will push your pH down. Have to watch that, too.