It's always best to use the smallest needle you can get away with. If you take the size 18 needle and push it through your fabric by hand, and notice how that feels. Then take a size 14 needle and push it through your fabric. Which one goes through easier? I'll bet the 14 would go through easier. A size 18 needle is usually over-kill. I wouldn't ever put one on a domestic machine. That's what industrial machines are for. When you get that big of a needle, you throw off your needle/hood distance, start skipping stitches, and put a nasty burr on your hook. If you're sewing something that thick on a Babylock with a plastic bobbin case, you're asking for trouble. The bobbin case will probably jam and the needle will go through the bobbin case, and you'll need to buy a new bobbin case.
Let's talk about thread over-kill... I imagine you're thinking you need "upholstery," "denim top-stitch" or "outdoor" thread to sew on something really thick. You don't. Just use a good Guterman polyester thread. It's made for your domestic machine, and is plenty strong. Those other threads aren't much stronger, and they WILL NOT give you a good stitch. They are made for industrial machines--although they don't say that on the package, because they want your money.
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