God, what's the obsession with inch per gallon rules?! It's not the be all and end all. There are other important things to consider like waste, present levels, space occpied, nature, nocturnal/day, filtration, maintenence and so on.
I would not add a bristlenose to that tank because of the waste they produce, not because of the size they become. I also reckon you should find out what the algae eater is before considering adding to the bottom. Add to that the fact that the bottom is smaller than an average tank.
Not a good idea IMO.