Say a multiple purchases were going to be made in a single day, say the purchases were of the amounts $500, $400 respectively paid by credit card on a card who's overall limit is $1000. If I was to move money (say $900) from a chequing account to the credit card, would I be able to make another purchase on the credit card (say $700) without encountering overage fees?
Yes, however you would have to wait for the $900 to actually be available in your credit card account if making the transaction from an account from another bank or provider, as it usually might take one to two business days for this to happen.
If both the chequing account and credit card account are with the same bank, then usually this will go through straight away, and you will be able to make your next purchase on the same day, but I would check your credit card balance first before making that purchase.
I can't speak for every credit card, but I know two of mine don't have overage fees. The transaction either goes through, or gets denied. Check your card agreement and look for the fee section.
One other thing to consider, sometimes when you make an online payment to a credit card, you will notice that the "Available Balance" number on the account will increase right away even if the payment is not reflected on your "Current Balance". If this is the case, and you are positive that your payment will be successfully posted to the account, I say go for it.