ClearAll[SpacesDistributeOverN, Possibilities]
SpacesDistributeOverN[s_, p_] :=
 Flatten[
  Permutations /@ (Join[#, ConstantArray[0, p - Length[#]]] & /@
     IntegerPartitions[s, p]), 1]
Possibilities[hint_, len_] :=
 Module[{p = hint, l = len, b = Length[hint], Spaces, out},
  Spaces = # + (Prepend[Append[ConstantArray[1, b - 1], 0],
        0]) & /@ (SpacesDistributeOverN[l - Total@p - (b - 1), b + 1]);
  out = Flatten /@ (
     Riffle[#, Map[Table[1, {#}] &, p, {1}]] & /@
      Map[Table[0, {#}] &, Spaces, {2}]);
  StringJoin @@@ (out /. {0 -> ".", 1 -> "#"})
  ]
Possibilities[{}, len_] := Module[{},
  {StringJoin[ConstantArray[".", len]]}
  ]
Possibilities[{2, 1}, 5]
Possibilities[{}, 5]
Possibilities[{8}, 10]
Possibilities[{2, 3, 2, 3}, 15]
Possibilities[{2, 3}, 5]