If you have already styled your CSS across the site, can't you just enter custom CSS into the registration page matching what you use elsewhere? I'm hesitant to remove my CSS because it is the CSS used by WordPress, however, I wonder if I could make my CSS a stylesheet, not inline, then it would be easier to circumvent. Failing that, I could make a checkbox to 'clear default style'. I'll look into one option or the other.
I'm suprised you are having a problem with the tab index. They should incriment from 21 to however many fields you have. 20 is the index of the user email field, so mine just follow. I had to do this because when any other plugin adds a field with a tabindex, if mine have no index then it will skip past mine to the other plugins then loops back around. Of course the problem with that now, if that if the other plugin loads before mine it's fields might literally be before Redux's and now it's me breaking tab order. Regardless, the only solution I see is to create, yet another, option, to set the first tabindex for Redux fields. Default with be 21, and it won't allow you to put anything lower, but you could analyze your other fields and set it to something like, 50 if someother plugin is creating a field 40. Maybe I'll let you enter 0 too to create no tabindex's... yeah.
OK so I'll add an option for tabindex and look into one solution or the other for CSS, long story, sorry, I just need to think these things through. I'll add both of these to the 3.7 roadmap, they are both simple so I can code them today.