You shouldn't expect the plecos to survive just on algae. Get some bottom feeder pellets for them. They need food. Also, that tank is much too small for two plecos, let alone six other fish.
Yes, but make sure that you have enough filtration for the bioload as plecos are pooping machines, and the plecos need to have driftwood in the tank to aid in digestion.
No. Plecos are classified as herbivores or plant eaters. They are also bottom feeders.
Tropical fish.
no
no plecos will die
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a catfish will eat any fish that is 3/4 its size.
Blood parrots are tropical fish, yes they do need heaters.
Home Depot offers tankless hot water heaters for around $1000 depending on the specifications you need. These will need to be professionally installed.
The fins are larger on the sailfin...but almost all the "common plecos" (Hypostomus plecostomus) on the market are actually sailfin plecos (Glyptoperichthys gibbiceps).