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I agree with the previous comments regarding doubling of the bass with the cello an octave above. This way the cello reinforces the first harmonic of the bass, not the fundamental pitch. It's like a glue that doesn't call attention to itself. Regarding relative loudness in strings … string players are incredibly malleable and 1st+2nd together can achieve the same pianissimo as 1st or 2nd separate. The texture may or may not be the same, but it's all doable. Your example was vague and could be orchestrated many ways. It would be helpful to have the actual music. One basic suggestion is to keep the intervals in the lower strings wide and open, and in the upper strings, tighter.

The violas, especially in an amateur string ensemble, are likely to be the weakest section (I can say this being a former violist myself). I would not recommend dividing violas unless in a very quiet part of the work.