When I place a limit or a stop order, does that then count against my available margin, or is it only if the orders are filled that it counts against my available margin?
Yes, limit orders and/or stop orders can be held against margin, but it all of course depend on the circumstances.
If you place a limit order or a stop order as an entry it will go towards your margin because there is a potential for a fill.
Another example: : you have been filled and you place another limit order, now you will have the positions that goes towards the margin and additional margin on top because of the potential limit to get filled.
If you are in the market in a position, and you place a stop and a target, you will not be charged additional margin on those orders because they are closing orders.
Let me know of any other scenario you are looking at.
As we explained above, pending orders could also be measured against margins.