You need 1 fishkeeper with a siphon or gravel vac for tank cleaning.
The species you keep in the tank should be based on what you want to keep.If you start keeping snails or plecos with the idea that they will keep your tank clean,you will be disappointed.
You have not mentioned the size of your tank nor its volume or capacity.
so its a bit difficult to tell. However a 50 gallon tank would be cleaned by one 6-8 inch pleco or 2 smaller specimens. On the other hand it would require about 4 -5 snails to do the same job.
as was said, rely on plecs and snails as nothing more than slight "helpers" dont expect them to clean the tank per say. however, they will eat SOME, not all, of uneaten and leftover foods your other fish miss.
depending on the kind of fish you have, have you thought about cory cats also?? after keeping them for a few months, i think their a much better addition than snails, for the fact you never have to worry about an over population of corys, or constantly picking snail eggs and babies from your tank.
If they're L029 Vampire plecs they eat meat and not algae anyway. Snails are a part of their diet & they get to 10" so I hope the tank is a 75 g or bigger. The only thing that cleans a tank is a python hose.
they eat some algea but to overall health of the tank common plecos are probably a detriment more than help, as far as snails go with your oscars they wont last very long before they are eaten . if your looking for algea cleaning fei fengs are awesome and pretty to look at