Another gauge question

The dd of a co-worker has asked for some purple and blue mittens (She’s 6.) I have some purple eyelash yarn (given to me) which I thought I would carry with a blue base yarn to make them furry. (Cute, huh?) But I have to purchase the blue yarn so…

Does carrying along an eyelash yarn change the gauge? It looks so fine my first inclination is no. My pattern calls for worsted weight, but if the eyelash is going to change the gauge, then I should probably buy a sport weight?? Offhand does anyone know the affect of a carrying yarn on gauge?

Any suggestions/recommendations? Whaddaya think?

Yes, you should probably buy a sport or DK weight, the eyelash will make it a bit thicker, so that will compensate.