a(6) = 4 is created by gluing two equilateral triangles along an edge to make a rhombus with perimeter 4:
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a(16) = 8 because 4($4) = $16 and the four squares can be arranged so the shape has perimeter 8:
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a(17) = 7 because 3($3) + 2($4) = $17 and three triangles can be built on top of two squares to create a shape with perimeter 7:
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a(18) = 6 because 6($3) = $18 and the six triangles can be built into a hexagon of perimeter 6.
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a(19) = 9 because 5($3) + 1($4) = $19 and this is one of the minimal perimeter shapes:
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a(20) = 8 because 4($3) + 2($4) = $20 and four triangles can be built on top of two squares to create this minimal perimeter shape:
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a(21) = 7 because 7($3) = $21 and the seven triangles can be built into this minimal perimeter shape.
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a(26) = 8 because 6($3) + 2($4) = $26 and the following shape minimizes the perimeter:
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a(33) = 9 because 7($3) + 3($4) = $33 and the following construction works: Take a triangle. Glue the three squares to its three edges. Use the remaining 6 triangles to make the convex shape of perimeter 9.
