I'm familiar with beta work, having done it before, but I'm not sure I want to take on the full responsibility for that. It's pretty demanding work on some levels when I've already got a project of my own I'm working on- what I don't have bogged down with real life, I have bogged down with games I'm running or participating in or fanfics I really should be working on. That said, I'm sure if you sent me drafts, I could take a look and give you a once-over- but I'm kind of a harsh beta, tbh. ^^;
I also really don't know if I should care about the 'modes'- I mean, that's the easy part. Most toku fans can come up with a pretty decent Rider/Sentai system/attack, with the biggest issue being that a lot of them c/p. I'd be more concerned with how you're writing the characters and overall story, since those things will make or break the fanfic for you.