Caps are different then resistors. Resistors, if you put two 1000 ohms in series you would get 2000 ohms, and in parellel you would get 500 ohms.
With caps it is the opposite, if you put two 200 microfarad in series you would get 100 microfarads, and in parellel you would get 400 microfarads. So you see, by you putting your 100 mic and 150 mic in series, you would only get 60 microfarads, thus not achieving what you wanted. But if you put the two in parellel, you will achieve 250 mic. So both +'s tied together and both -'s tied together should do the trick.
On another note, when replacing caps (or your building your own project), do not ever go under or above the value labeled in the schematic (or what you calculated). And do not ever put one in with a lower voltage unless it still exceeds the voltage present there. But you can always put one in with a higher voltage rating.
I hope what I have told you had answered your questions.