I'm glad to hear they aren't congo tetras
. Though even X-ray tetras need 24 inches swimming length, as do harlequin rasboras.
I do 50% weekly water changes on both my tanks. It won't harm the bacteria because they don't live floating in the water - they live in the biofilm which is tightly bound to surfaces, and other bacteria live in the substrate. But large water changes do remove all sorts of chemicals excreted and secreted by the fish.
Topping up should always be done with pure water - something like reverse osmosis or distilled water. That way you put only water in the tank to replace evaporated water and not water plus minerals.
I have never kept bettas with any other fish. I have had about half a dozen bettas, one at a time, in a 6.5 gallon tank. They just aren't community fish, no matter what you read. There are so many websites out there written by people who keep fish and don't necessarily know what is best for fish. Seriously Fish is the most reputable site and that also recommends keeping them alone.
I have just noticed that you keep the temperature at 74 deg F. This is a bit on the cold side for bettas - they need 76 to 82 deg
http://www.fishforums.net/threads/betta-splendens-caresheet.423362/
Can I ask - what species is the catfish? If it's a cory, they are shoaling species and need to be in a group of at least 6.