As long as you know it's not really your money, you should have no problem.
As far as credit ratings and scores and limits go, you will do the best for your credit rating if you carry a balance that is about 20-25% of your available credit. It shows that you are a responsible borrower, but one that the bank will make money from. So maybe you could set your limit so that you will use the card to that 20-25% amount occasionally and then pay it off.
People who have a lot of available credit but never ever use it don't necessarily have better credit scores than responsible borrowers.