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someone at petco told me that if i get a protein skimmer for a 60 gallon, and use it for a 29 gallon fish tank, then it would damage and ultimately kill my fish. Is this true?

it is a berlin airlift 60

A berlin airlift will barely skim a 10g tank effectively - it certainly will NOT overskim a 29g.

and I've never ever heard of overskimming killing fish (nor can I deduce the assumed methodology - over removal of trace elements perhaps ?)

Its possible that using a huge $5,000 Deltec skimmer on a 29g tank would keep the water too clean for some corals (elegance corals and xenia come to mind) but I can't picture any other situation where a skimmer will kill livestock.

A slightly oversized skimmer might strip out phytoplankton, and this would be bad if you were dosing phyto to feed featherdusters, sponges, and clams, but even then it wouldn't directly kill them (just waste your $ spent on the phyto).

Personally, the money spend on a airlift 60 would be better invested on a powered hang-on skimmer - your going to be spending a fair amount of extra $ on a big airpump and wooden airstones, plus frequent tuning... and since that model is co-current rather than counter-current (I think) and fairly short - its going to be pretty inefficient.

(this isn't a total slam of air-driven skimmers - I think a big 6ft air driven is a great DIY option, but short air-driven skimmers don't do much in my opinion.)

Oh - and as a person who moonlights once a week at a Petco (somebody has to point folks to Neds...) I know for a fact that the quality of the help varies (both between stores and in a given store during the week) -- Burlington has a really good fishgeek, Norwood used to, Taunton reportedly does, and they say the guy who works Sundays in Avon is ok -- but I have also heard horrible advice from other employees in those same stores ...

I also know for a fact that Petco doesn't sell a skimmer that I would "heartily" recommend for anything bigger than a 10g tank.

Skimmers, like, activated carbon, for a reef tank
are best used only very occasionally.
Certainly do NOT use either while feeding your DT's phytoplankton,
the skimmer will _ it right out.
A skimmer is useful if you have a lot of fish, or large fish that you feed alot.
Basically a water change does the same thing as a protein skimmer.
Also, if you are using a large skimmer, or
if you do use a skimmer 24/7,
check your salinity OFTEN, because those littles bubbles 'o' goo
in the collection cup IS saltwater.
Also a skimmer can take out certain nutrients and possibly trace minerals, so things like strontium & iodine may need to be added more often.

in stock we have
Red Sea Prism skimmer for $140
this would be plenty for a 29 reef...
we also have a CPR DUAL PAK Bak pak for $195...
which is more like for a 75 gallon reef tank