They are not released yet, so, anything anybody says in reply to your question is simply going to be a guess. Guessing is pointless.
Based on what we DO know so far, MOST boards WILL need a BIOS update. Some boards that have or will be released AFTER the compatible BIOS versions were available, or boards that specifically say they are ready for Ryzen 5000 series, won't, but those will be few and anybody who already has a board right now, WILL need a BIOS update to support Zen 3 CPUs.
Both chipsets have the same form factors to choose from. Not sure where your confusion on that stems from but pretty much ALL motherboard chipset families except maybe some enterprise or HEDT parts come in mini ITX, microATX and ATX form factors. So it doesn't matter if it's a B550 or X570, there are a variety of form factors available.
As far as being a "better" alternative, that's probably just a toss up. Either chipset is pretty equitable and to be honest, B550 seems like a complete waste of time, money and resources because unlike past budget chipsets, these are NOT "budget". They are just as expensive as X570 boards in most cases except on the very lowest models.