Re: redecking
The resistance to twisting will come from your extra stringers. To help with diagonal racking weld an outer frame onto the stringer ends. If the decking option your looking at is .090 (about 3/32") and 2"x2" channels for stringers then my best advice would be to go with 8"on center,which would leave you only 6" to bridge with the sheeting.
My deck and frame ended up very stout on 12" centers,but I used 1/8" diamond plate which has the strength of 3/16 plate. I think you will feel the decking give without tightening up the gap on .090. Going from 12" to 8" will add another 11 stringers. I also used stainless self tappers on 12"x12" centers to utilize the strength from the sheeting. On .090 I'd up that too at 10"x10" centers. I would also score some 1/4" backer plate for mounting stuff like pedestal seats.
As far as grip on the sheeting I'm not sure what I'd use. Whatever you use for a 8'x24' deck that will stick to aluminum wont be cheap. There's suppose to be a way to make line-x stick to aluminum,which would make a nice surface. You could also lay vinyl on it. The extra stringers and coating and screws would probably cover the cost of the diamond plate,if all you need is a fishing platform.