Uh, well, if what you want to do is to show off your skills in one particular mode, then maybe not do that in an all mode tournament? The tournament is supposed to show your prowess in every mode, not just the one you specialize in. If you think taiko and standard is too hard, then strategize accordingly when you pick maps during the match.
Are the mania maps too easy? Dunno, it seems like only 7k is starting out at normal difficulties for the first round, which I'd say is fairly appropriate all things considered; it's harder to manage 7 keys than 4 at the same time. 4k, CtB and Taiko seem to be closer to the hard range of things. I guess I could agree standard might have harder maps overall compared to the other modes, but the main mappool are also maps you can play as converts. I haven't looked into how difficult those are in other game modes, but they might balance it out better.