Ionic or covalent? Are my answers right?

Polar substances (e.g., salts) are generally very soluble in water. Salts use ionic bonding. Non--polar substances (e.g., organics like er and fat) are generally not soluble in water. If a metal is combined with a non-metal, then it is most likely ionic bonding. I think you are right in this respect about the bonding. You could look at the particular lattice structure of each substance and look at the strengths of the bonds between each atom to make a guess at solubility. You could make a good guess though if you know that a substance involving the same groups is soluble or not. For example, CH4 is not readily soluble in water so SiH4 is most likely the same.