Well for the pleco, you'd have to know their maximum size. In a 5g with platys(who will easily reproduce if you have both genders), a "common" pleco that you normally see at pet stores is not suitable for the tank. They can easily get to 12" or bigger. There are hundreds of types of plecos, such as clown plecos, bristlenose plecos, royal plecos, zebra plecos, galaxy plecos, and green phantom plecos.
You might want to try researching at http://plecofanatics.com/ to look for a pleco that gets no bigger than 3" in size.
For snails, only certain snails will "breed like wildfire." Some snails, such as apple snails(golden inca snails, black mystery snails) have a specific gender and will not reproduce by itself. Also, they lay a clutch of eggs above the waterline, so you can always quickly remove them if you don't want babies. Other snails like pond snails, ramshorn snails, and malaysian trumpet snails will breed by themselves and will invade your tank.
For the loaches, most(if not all) will to be too big for the tank. Loaches like clown loaches grow extremely slowly but eventually get big(12"), like to swim in schools of 6+, and are better for eating pest snails rather than algae. Weather/dojo loaches are coldwater fish, despite them being placed in tanks with tropical tanks all the time. They actually prefer temps around 65 degrees, which isn't compatible with platys.
I would just get 1 or 2 apple snails, since they don't take up too much space and have a low bioload. If you find that you have too much leftover food, try feeding them something that floats, so the food doesn't get trapped under the gravel. Or you can feed small amounts multiple times per day so the platys find the food everytime. Keep in mind that although "cleaners" will help eat uneaten food, the food doesn't disappear inside their stomach. The more they help clean up, the more waste the produce, which in turns still lowers the water quality. The best solution is to have good filtration and water changes with water conditioner, and to minimize the amount of leftover/uneaten food.
If you have an algae problem, you MIGHT want to try getting fast-reproducing snails like malaysian trumpet snails. If you can't find them at a local store, you can get them free as long as you pay shipping here: http://www.aquariacentral.com/forums/showthread.php?t=194326
MTS only reproduce based on the abundance of food. If you don't have any leftover food, they'll be competing with the algae for nutrients. Their population will be kept in check by the amount of food they have(although you should still expect to have a lot of snails).
Good luck
