I'm doing some creatine, HMB, and also increased my protein via pea protein in my smoothy post hard weekend rides. I was doing 2 teaspoons of creatine with no ill effects and then my wife read that 3 was better which caused some GI distress but tolerable. The science says it is helpful and can't hurt. I'm on a 95% plant based (vegan) diet which provides enough protein for a normally active senior but not enough for those of us always training and preparing for the TDF.

Wrt peds: I do like me some caffeine during a long ride. We have a nice espresso machine and I sometimes bring along a Cafe Cubano. Caffeine infused gummys from Bonk Breakers are also good as are some of the Cliff Bars.