if you are renting I can see where you might need some special considerations as far as keeping things pretty temporary and might even have some aesthetic considerations. building a tractor large enough for 12-15 birds is possible. I think getting one lightweight enough to move often and having an appropriate size at the same time will be the trick. dont' get sucked into the pictures of tractors packed with birds. yeah some people do that but it sorta defeats our purpose in trying to get some greens and provide good living conditions for them. for instance in this picture I found ......http://www.lilligren.com/homestead/pix/pasturedpen.JPG
i can count at least 50 chickens. I would cut that in 1/2 at least and probably more like 1/3 to 1/4 just so i didn't have to move the pen twice a day.
I think my next tractor will be similiar to this.
http://www.pvcplans.com/pvc-pastured-poultry-pen.htm I can see already in that particular design that it needs some cross pipes at an angle for stability. there is a variety of pipe fittings that could accomomodate that need. pvc would need to be painted to protect it from the sun also and keep it from deteriorating.
I differ then alot here. I am not a huge fan of portable fencing. I would rather move a large tractor daily rather then mess with moving portable fencing. electric is ok but you have to have a good ground and a good charger access wherever your moving your fence to. this can be limiting as to where you can move your pen anyway. or leave you with a long run of electric fence or insulated wire going across the ground to your new desired paddock area. to me a large tractor is more versatile. It can go anywhere there is open somewhat level ground, be moved by simple picking it up and rolling it (no pulling up an moving lots of step in posts and tangled fencing, which having just remved a large portion of electic fence I can attest to being tedious and time consuming) and if desired can serve the exact same function. If one were trying to raise large numbers of birds then I can see where a large paddock/pasture system or movable fence is neccesary. building a tractor for 100 chickens just doesn't seem reasonable.