Hi geordie2011
i would say the racking would support the size of tanks your after with no problems, when you say 12 tanks i assume you are saying 4 on bttm, 4 midd and 4 top. the tanks on the bottom are more or less sitting on your garage floor you would just need to pack shims under the bottom shelfs so your first 4 tanks are not actually putting any weight onto your shelving. The tanks on the middle and top shelfs are your loads on your shelving, as i said previously for added strengthening you could bolt all the back legs into the brick wall of your garage and you could also bolt the cross members which join each shelf to the legs to give an added stiffening point for each level.If your worried about sagging in the middle of each tank you could add en extra cross bracer in the middle or use thicker wood for the tanks to sit on.
Regarding your other question about a tank in an upstairs bedroom as long as its sitting across the floor joists and you say its a 3ft tank i see no problems, but i wouldnt go wasting money on a steel bench, an ordinary cabinet thats with any fish tank would be fine