I've seen this cage in person and can't believe that the big green place sells it for any animal... but then again they are like that -.-
As a temp fix that is a very good solution and because the wheel is too small then he is better off without it for a little while rather than having one that could damage his spine.
Focus on getting the cage first and then look into getting him an 8" or 11" wheel. The minimum hight of the cage will need to be around 50cm just so that a big enough wheel will fit in and as others have mentioned the length and width of the cage should be around 80x50cm or more if you have the space for a bigger one. Look more towards rat cages rather than hamster ones as most shops sell laughable "Hamster Suitable" cages.
I have a few decent sized cages linked in my website - Bobtail Hamstery (linked below in my signature) but the housing section here on HC is a great place to start getting some ideas for the kind of cages you will want to be looking for
As for the nuggets for your robos I don't know many people who actually do feed nuggets and at the same time I don't know anyone who has had problems with one ham eating the better foods. To be honest in a decent food mix the majority of the food is great so it's not something to really be concerned about. Take a look at weaning them onto Harry Hamster or Burgees Hamster food - these are both great and have high nutritional value. I don't know the NV of nuggets but I'm sure you can find somewhere to compare them online
Your hammie is beautiful by the way