Yes you can use circs for flat knitting. I do it all the time. I have not learned how to knit in the round yet. Knitting on circs as opposed to straights gives you the same effect as knitting on straights. There is NO difference to the FO.
I love to give homemade gifts... which one of my kids do you want?
Find our "Canadian Oddball Blanket KAL" on Ravelry as: OddballBlankets