Welcome to TFP!
It is possible to use a smaller gas heater on an above ground pool, though there are a couple of things you need to watch out for. First, you need to make sure your current pump has a high enough flow rate to keep the heater happy. The smaller Index pumps aren't usually enough. Second, you need to run rigid pipe for at least a couple of feet from the heater. Otherwise you run a risk of the heater melting the hoses.