You can certainly keep multiple parrotlets - but you're much better off having each in their own cage, so they can 'talk' to each other, but not get territorial with each other (unless you are trying to breed, of course). Chances are high they can share 'out of cage time' together, but make sure it is always supervised, just to be safe.
All parrots 'can' screech. But a parrotlet screech is more like a shrill human whistle than the eardrum-splitting, headache inducing blasts a conure or, gods forbid, a 'too, can produce. I've only had girls so far - apparently boys can be a bit louder - but they rarely got above a relatively pleasant chirp, even when they were complaining about something (like me leaving the room.) Their normal little 'talking to myself' sounds are actually so soft they're hard for me to hear XD (But to be fair, I have substantial hearing loss.)