I just tried to build a rather tall rocket which also uses several Quad-Couplers to add more engines, and as you can see in the screenshot below this thing is rather floppy and unstable:
It also doesn't fly far due to these problems. I tried adding struts, but at the critical points between stages I can't seem to add any as the Quad-Couplers are wider than the fuel tanks.
How can I build tall rockets that don't flop around so much and that are actually spaceworthy? I'm both interested in this specific case, but also on general guidelines for this.