It depends how small your cage is
It may be fine for a starter cage. Robos are very active though so they do need a good amount of space. However they also feel nervous of open spaces so the set up needs to have lots of toys/hidey places on the substrate to provide overhead cover and a shelf ideally (to sit under).
The main concern with tiny dwarf hamsters is fall risks if the cage is too tall.
String isn't good no. A wheel shouldn't fall over if it is placed on a flat surface - if it's sat on the substrate it might. You mentioned bars though - I think it's possible to attach a silent spinner to bars? Without the stand. Do you find it spins ok? Because there have been issues with them not spinning properly so many people end up getting an 8" wheel.
Our robo has a flying saucer, which he loves - the 7" one. That won't fall over when sat on the substrate. He also has a 9" wheel screwed to the bars
And he uses both!
Yes as Cypher says, they are absolutely fine on their own - in fact they should only be kept in pairs or groups if they are siblings and have always lived together - most people end up having to separate them anyway due to fights breaking out and then you need two cages.
Which cage is it? Ideally you would need something with at least 70cm by 40cm floor area and no taller than 36cm. If it is taller than that it may need a full level making (or a lower cage buying if the floor area is too small).
Our robo is in an 80cm by 50cm cage and very happy in there - plenty of space for his two wheels and he also has two houses/burrows and nests and moves between them (ie if I disturb one he moves out and uses the other one!).
Main thing is don't overclean - it causes a lot of stress. Put as much substrate in as you can (they like to dig and burrow under the substrate) . Weekly cleans aren't necessary - just spot clean as and when needed mostly and then you don't need to change all the substrate for about 6 to 8 weeks. Don't clean everything at the same time either. If you do the substrate then clean the wheel another week and the toys another week again. Then something always smells familiar to them.