Headset and bottom bracket must match the frame. No getting around this. Do a "dry run" with the cable and housing routing before making any final cuts. Make sure you did your homework when it comes to bottom bracket axle length and chainline before any derailleur setup takes place. Don't assume seat tube diameter or stem clamp diameters-measure them. If it's a new frame or freshly painted frame, make sure there's not paint build up in the dropouts or the end of the steer tube and bottom bracket shell that may cause fit problems. If possible, check the alignment of the dropouts and derailleur hanger. This could possibly save you time trying to diagnose a shifting problem. If it's a road bike, don't wrap the handlebars till you've had a chance to get in a couple rides to get the hood placement dialed in.