thanks for such a fast reply Bunny ^^ it's helping us help you that you're so quick.
the five gallon tank should be fine with the heater, but the amount of fish spells trouble! from what i gather, two Platties (2"), one Guppy (2") and a Molly (4", i do believe) are much to much for the tank to handled. perhaps returning both Guppy and Molly and replacing them with another Platy is best. be sure you have good numbers for your Platties; two females to a male is good. females have a fan shaped fin, males have the more streamlined altered 'boyparts' instead of the anal fin.
i myself am fourteen, but i work volunteer hours at my LPS so i get lots of discounts and free merchandise
Ammonia kits are often between $7 and $11 dollars Canadian. Nitrite i got free, and Nitrate i still need to get my fins on. since you can't afford the test kits, just do water changes. i was one of the lucky 'Dumb-Newbs' as my fish didn't die; all four Kuhliis and the Betta survived a cycling period with minor Ammonia burns. if you change maybe a gallon of water every other day you should be able to keep everything in check.
as for the compatibility, the only time i kept Guppies i was five and kept taking the Kuhlii out of the tank and calling him my Wormy Thing, poor thing didn't live too long. if the Guppy is a male and the Platy he's chasing is female he might want to breed with them, and it could end up the same in the Mollies case!
i'd suggest calmly asking your parents for an advance in allowance or a loan, and tell them you like your fish but they might die if you don't get some proper test kits for them, and that would mean more money for new fish instead. if you can get this money off them, try and get the product 'Cycle', it's one of the only chemicals i think works when it's used together with fish food, or in your case fish poo and food.