I would run the test that fits your tank. If the pH isn't in the 6.0-7.4 range, there is no reason to run a test for that. If, however, after running the high test, your pH comes out at the lowest end of the spectrum, then I would test it with the lower kit to make sure of where the pH actually is.
I understand that you want to trust the guy at the LFS. All I am saying is that you need to have done your own research before you go to the store. Being right about one thing doesn't mean that they will be right about all the things you need/want to do. I'm glad you didn't get soaked for the large bottle, but I'm not sure that you didn't get taken with the smaller bottle. You have nothing to worry about with the mature media being in your filter. That stuff is already established and once it acclimated to your water conditions, it will be fine. It takes a good bit to through a mature colony completely off. The newly growing colonies are a little more finicky, but the presence of the mature media should do just about everything you need it to do, including cover up for the other colonies should they hit a bump in the road.
I don't want to come across as negative, but I want to help you for the future. You don't need to spend even the $6 if you have mature media to pull from (which of course you now will because you have at least two tanks running). If you decide to start another tank, don't buy another product, just pull some media (never more than 1/3 of the total amount) from the established filters and put that into the new filter. Then, replace the media you took out in the old filters. Doing this will ensure a smooth transition to the new tank, and it will not cost you time in dosing daily with a product with marginal, at best, results. Aquarists have been using mature media to seed filters for generations. It doesn't cost anything more than replacement cost of the material moved. It will instantly cycle your new set-up for a proportion of your old set-up's bioload, and you won't need to dose daily with anything. Fish can be put in the tank, immediately. Of course, you will want to test for a few days on both tanks to ensure that things are fine, but experienced aquarists know how to handle this process. Generally, they don't feed the new or old tank for at least a day. Then, they feed sparingly on both tanks. As you know, more food = more ammonia. After three days, the colonies are up and working at full strength and the new tank is starting to colonize the new media.