Just a reminder, since it's still kinda new on the forums here (and largely untapped): we have
competitive tutors here now, myself included.
I'd be more than happy to sit down with you (metaphorically ofc, and all of the other tutors are available, too) and discuss things 1 on 1 and one at a time.
I personally experimented in all of the singles format tiers as of gen 5 (not too much changed in gen 6 for lower tiers) and in gen 6 specialized in singles OU, UU and Ubers - plus delving into VGC '15 and '16. I am also very experienced with making custom sets that at the very least rival the super-common sets seen in competitive play, so everything need not be 'by the book'.
I also want to point out that OU, UU, etc are essentially based on usage - which makes sense considering OU (over used) pokemon tend to be the most powerful, versatile, and/or have the best coverage options while UU (under used) tends to be slightly less so, etc. Also just because a pokemon is listed as being UU (for example, Umbreon) doesn't mean it's unfit to be used in higher tiers such as OU (in fact, one of my Umbreon sets regularly walled entire OU teams in late XY and early ORAS). Of course, lower tiered pokemon tend to be harder to use in higher tiers (hence their lower usage → lower tier), but the only part of tier classification that really means anything is that higher tiered pokemon (such as Scizor which is ranked in OU) are banned from lower tiers (such as UU, RU, NU, PU, etc. in Scizor's case).
Also note that LC (Little Cup) has no bearing on the other singles formats. I can explain more if you're interested, but LC has to do with the pokemon being the first evolutions of pokemon that can evolve and not usage statistics. it actually one of the most balanced formats I've ever played - though most people seem to avoid it, incorrectly assuming everything there is 'weak' or that they wont have many options.