i think it would work x4 Blood Parrots and x1 Pleco. Even if its that way for a while and the Pleco gets to 12" and the x4 Parrots are about 6-7" each, thats about 38", only 0.5" per gallon
You will get various magic formula's thrown your way, about how one parrot needs 235gallons and two need 470 gallons (i jest, and exaggerate), but fish keeping is not so black and white and these fomula's are next to useless if standard good fish keeping practice is adhered to, by this i mean (and assume you will be)
- Over filtered, in terms of really maximising your filtration, i would expect one goof external on that 75 gallon as a minumum and ideally i would go for two, as its nice to have some back-up (i know money could be an issue there).
- Water changes, looking at a minimum 20% weekly, i would do more
- If you keep these fish for most of or all of their life-time you would consider upgrading to a bigger tank, especially if you think about adding any fish to the x4 parrots and x1 pleco#
- Unless you really experienced, i would do some water testing, especially for the first few months, until you have everything nice and stable.
These are just my thoughts, i guess on my own idea of "good practice" others may differ, and i have only highlighted some of the basic's there.
For quite a while i have had x8 Blood Parrots in an 80 gallon with a few other fish, i am not moving up to a 145 gallon, as i intend to keep this fish for the rest of their life, i absolutely adore them, but wouldnt want any of the dyed kind. They are all bright orange, ranging from 4" to 6" presently.
If anyone says the parrots "might" not get on, or it might get too aggressive, well yeh, thats the same for virtually all cichlids, but i would not advise against that stock just in case it "might" happen. I think four is a decent number and any aggro should be spread out amongst them to stop one fish getting bullied.