Re: Training TKD/Karate while taking aikido?
I've got some years experience in both TKD and karate (shotokan), so that's my perspective. If your goal is fitness, create a fitness program for yourself, or get a trainer to design one for you. If that's your goal, it's not a "waste of time". Instead, it is using your time to do exactly what you say you want.
See, this is the problem with the reasoning that says that two styles are twice as good as one, or that depends on some hypothetical advantages of one style or another based on a cursory and generalized understanding ("higher caliber of an instructor" (presumably because he has a higher number next to his dan ranking...in a completely different style...um, right), "you need a large amount of balance", "it increases flexibility", "50% balance lost", "karate just seems like a more complete system", etc.). In real life, every one of these seemingly logical arguments is quite likely to be completely wrong. As a generalization, however, I'll make two observations:
1. Because of rank inflation and focus on tournament sparring, it is indeed quite hard to find a good TKD school these days. However...
2. Based on a google search of "John Sharkey shorei", he would not be my choice. Mind you, I have no personal knowledge of the guy...but I tend to avoid self-proclaimed "Master"s with an 8th dan who say nothing about their lineage, dojos where the students wear gis of varying colors and belts with stripes, multiple schools run by the same instructor, schools that claim to teach four dramatically different styles (including something called "Xtreme Martial Arts" which is apparently "a combination of techniques, methods of movements, and philosophies from all martial arts styles blended together with high-flying acrobatics, gymnastics, and the hottest Hong Kong "Chop-Saki" action").
A plain old gym looks like a pretty good option.