I think you should probably refrain from talking to the people at petsmart for suggestions. White stringy poo can be indicative of a parasite or infection and fish get stressed out and sick VERY easily. You should definitely err on the side of caution and feed your pleco VERY sparingly until your tank is cycled.
Have you read through all of the stickies on this site? There are a ton of them about cycling a tank and things you should know when starting a tank. Doing a water change every day (making sure to turn your filer off while doing the change, adding dechlorinator/water conditioner and not putting water in the tank that is a drastically different temperature...) will be a great start for keeping your algae eater as comfortable as possible. When you get the money together, you should buy the ammonia test kit, then the nitrite test kit, then the nitrate test kit. Those three are REALLY the only ones you need to make sure your water is safe for your fish.
Don't feel like a failure or get down on yourself, most of us (if not all of us) have trusted what a fish store employee said to us, only to find out it wasn't even close to true and definitely not helpful. Finding people who are passionate about fishkeeping like the people on this board is a step in the right direction
Once you get your tank cycled you should look up harlequin rasboras...they are one of my favorite smaller fish