i dont know about laws but ive looked at two temp rentals in the last two days and can tell you what they are asking for- usually its six months and you have the option to renew.
a couple are doing one monthers with 20% of the month's rent as commission for them. the six monthers tend to be asking for a month's worth of rent as their commission.
either length of temp rent asks for the first month and a month deposit. some are asking for like three months up front.
if the rent is listed in dollars, so far in my experience, they will let you pay in pesos at the rate of 6 to 1, so thats great.
i have had the best luck finding agencies by walking around the areas i like and just talkin to them- there are inmobiliarias on seemingly every street. also going to clarin.com's real esate and enbuenosaires.com . i havent had the best of luck on soloduenos but was thinking of tryin to contact some that are on the traditional rent pages and asking them if they want to do six months up front, and then try to get an escribano to write a good contract. but i kinda think anythin written here may as well be smoked upon signature...suerte!