Gauge swatches

OMG. I am in a charity knitting group at my church and this month we are doing chemo caps. Well I started one already in Peaches n' Cream Peppercorn (I think that's the color name) and it's coming out great. I was at AC Moore's today and I found this very, very very soft yarn. It's Lion Brand Micro Spun. It's a microfiber acrylic but very very soft. The color is called leaf and it's a pretty shade of green. I love green. I found this book called Knitting for Peace written by Betty Christiansen and there is a chemo hat pattern in this book with an eyelet and it's very pretty and I was in the store saying to myself. Self that hat would like very pretty in that yarn so I bought it. This is my problem, the pattern states that I need to make a gauge swatch with circular needles but I didn't want to buy them before I knew what size I needed. I only have size 9's (dp) and 2's (dp) but I was going to get a 16 inch circular for this project but I need to make a swatch first. My question is can I make a swatch with straight needles and then adjust it to circulars? I will have to research this. I know someone from the library knitting group that can help me. I'll ask her. Talk to you soon.