I like the idea a lot (indeed I made one similar using a Guijia) but it has a glaring issue: the smaller arms don't match at all with the position of the larger arms. In Appleseed the pilot puts his arms in the smaller set and as he moves those around, the larger arms copy the movements exactly. When I made slave arms for my Guijia I used Eldar Guardian arms which match pretty closely to the position of the Guijia model. You might be able to adjust the marine arms to match.
Aside form that, I heartily approve of the conversion, looks great.