I don't think Thomas becomes a Winter Knight. Either a KOTC, or Summer Knight which could could create some problems for Dresden as Winter Knight
I could see Thomas becoming Summer Knight and that creating problems for Summer and Winter, who would then cause problems for Harry and Thomas, who would then cause problems for Summer and Winter ad infinitum.
On Harry's level of training/experience/competence or what have you, I'd say it's more a difference in emphasis than degree. As pointed out Harry's got/had Bob. This probably gave him information that most wizards don't have easy access to. He was trained in surprise close range magic combat without any focus items (blasting rod, etc.) by Mab. He's directly, deeply, and studiously examined Merlin's handy work. He's had lessons from a fallen angel.
But we also know that Harry is missing a lot of apprentice time. He was the youngest wizard for years. Eb barely taught him any magic. Harry had, at most, an eight year apprenticeship. (I think Justin took him in when he was 10. Eb taught him until he was 18). More reasonably 6 years. I'm guessing most wizard apprenticeships last into at least someone's mid-20's. We know he wasn't taught certain things that are traditionally taught. Carlos talks about how "Old Ones" are "the last thing a formally trained apprentice learns about." He mentions that Harry's training wasn't formal.
Side note. Carlos says Old Ones are Outsiders that are inside, but inactive. ("caged, bound, and sleeping").