welcome to the site, it sounds as though you have a great start in the hobby. I too started out with a 29 gallon tank, then upgraded to a 55, and soon to be bigger, as my common pleco, and my spotted highfin pleco will out grow even my 55 gallon. As far as breeding the platies, dont worry, they should breed on their own. Live bearers are some of the easiest fish to mate, just keep a watchful eye on things.
Again, welcome to the site, how big is your pleco? The reason I ask is because he will get too big for the 29, as they can reach lengths of over 1 foot long...