not true...
A longer cage won't adversely affect the shifting.
I would buy a Campy Veloce or Centaur cassette rather than an offbrand. Nashbar used to have them pretty cheap.
The facts regarding the use of a short cage with a 13-29 are as follows. If you increase the chain length by one inch (requires a new chain), you should not have a problem if you shift into the 53/29. With insufficient cage length, the chain will then hang loose in the little ring and one or two of the smallest cogs, making them unusable. A medium cage or long cage RD will make all the combination useable.
Using a short cage RD with the 13-29 will exceed the wrap capacity, so it's a nonstandard setup. Using the normal little ring/little cog setup would suggest a chain length that's 1 inch too short. Here's how to adjust the chain length properly.
Two simple tests will determine if the chain is the correct length. First, it must not hang loose in the little ring, little cog combination. If there is no tension on the chain in the little ring, little cog combination; remove two links (one inch) at a time, until there is. When the ends of the chain are brought together, some movement of the lower pulley should occur, indicating tension is being applied. Two more links (another inch) may need to be removed, beyond the point of absolute minimum tension, to keep the chain from rubbing on itself or the chain guide tab as it passes under the upper derailleur pulley. If you want to see how much lower pulley movement will occur, without removing the extra inch of chain, shift up four teeth (11 to 15 or 12 to 16). This has the same effect as removing two links. Once this is done, the chain is set to the maximum useable length. Removing additional links will do nothing but reduce the derailleur's capacity.
Second, the chain must be long enough to avoid over-extending the rear derailleur when shifted to the big ring and biggest cog combination. If the chain is set to the maximum length as described, it should always pass this test, unless your setup exceeds the derailleur's stated wrap capacity. If you deliberately exceed the derailleur's capacity and the derailleur is over-extended in the big ring/largest cog combo, then you must either avoid that combo or add another inch and avoid using the little chainring and the smallest 3 or 4 cogs (since the chain will hang loose).