Is the habitrail ovo a good cage to add onto another cage?

hey! yes, that set up should be fine for the hamster. they need more room than the habitrail gives as you said. for the pee in the corners thing, mine always did and do. i have my current hamster litter trained so he goes in his little "potty". it works great. i would try litter training. it is really easy. i bought my potty and litter at petsmart but you can use something like a plastic lid and some bedding. all you have to do is put it where he usually goes and stick some of the wet stuff into the lid. after he starts using it, place the lid where ever you want it. if he doesn't catch on the first time, just add a little more "used" bedding to it. he'll get it eventually.