Wow, 15 amp hours is a lot of capacity for two T100s! What kind of run times are you hoping to get?
You are correct that LiPo batteries are a good choice for the T100 and T200 thrusters. For maximum efficiency, 3s 11.1v LiPo packs like the one you linked are recommended for the T100. That Floureon pack is fine, but you can get better batteries for cheaper, notably from HobbyKing. They have a handy LiPo battery finder tool which will point you to batteries in the size, voltage, and capacity you are looking for.
I’m not sure what size limitations you have, but the highest capacity 3s LiPo they offer is this 8 amp hour Zippy pack for $44. Two of these in parallel would give you 16 amp hours of capacity, and be much cheaper than three of the packs you linked. The higher 30c constant discharge rate may give slightly better performance under load as well.
We haven’t tried this specific pack yet, but we are using an array of Zippy, Turnigy, and Multistar batteries from HobbyKing for our testing at Blue Robotics without any problems.