Hello Goal46. So yes, your original plan of paying off all of his revolving lines of credit -- but keeping the Cap 1 card at a smallish balance -- is great. In fact, you can pay off all of them completely, which will prevent you from paying interest. Just be sure to buy something on the Cap 1 card after you pay it to $0.
There's a big range you can have that Cap 1 card reporting at. Given the credit limits as you have described them, any balance between $10 and $300 would be fine. Just let the statement post, and then pay whatever was listed on your statement in full a few days after the statement prints. And make sure that there's a positive amount in the $10-$300 range on each statement.
When do you think you will be buying a house?