I don't tend to mix different patterns up (because I'd only lose the bits) but I do alter patterns to my own taste if I want to. For example I'm wearing the Merchant & Mills Factory dress pattern today but made it out of jersey fabric, omitted the collar and made it a scoop neck, raised the waist and lengthened the whole thing to reach my knees because I'm tall. I did my usual alterations of adding a bust dart as I'm busty.
Then I faced the front opening placket with Liberty lawn and added matching Liberty cuffs to the short sleeves. Looks nothing like the original pattern now but I like it better than the original look
Sometimes I use a tried and tested short sleeved pattern piece from a McCalls pattern to add sleeves to sleeveless patterns though - as long as they fit the armhole it works very well.