I have a Luguna Power clear 1800 rated for a 1800g pond on my 180g. The recommended flow is 700gph for algea kill. I run an Eheim 1250 through that UV at about 350gph and I have found that to be just about perfect. Water is always clear and I never have parasite issues. My point is, under powered/flow UV is the key if you want to run a lower wattage UV. If your setup wont allow you to run an independent UV system and you have to use the main filteration system, a higher wattage UV should be used. OP, I wouldn't have any problem using the in-tank type of UV, but if it is rated for your tank size, don't expect much pathogen kill. I would use two together.
Just my opinion.