Welcome! I’ll try to answer your questions in order.
Characters max at 0xFF. If you want to extend that you’d need quite an overhaul. You can extend classes to 0xFF (default is 0x7F), but you need some form of save data expansion patch - there are several available. I’m not sure if you mean supports as in characters to support with, or number of conversations per save. If the former, then nothing exists publicly that I know of. You can read more about it here, though it’s pretty technical so judging by what you say of your skill level it may not be of much use.
A lot of what you describe here is dependent on the length of the vanilla tables - I’m not sure how familiar you are with hexadecimal, but the same code used to read up to 0xCD (the vanilla item table length) could be used to read values up to 0xFF. Going from 0xFF (the length of the vanilla character table) to 0x100 would require an extra byte to be read and thus the vanilla code would fail to do so. In essence, expanding one is as trivial as adding more table entries while the other would require large amounts of system overhauling.
Understanding what you’re looking at in memory is very dependent on context. The easiest to understand by humans is tables since values are rather easily read and interpreted. I am unaware of any guides on the subject in particular, but the given memory locations by FEBuilder are an amazing tool for learning. Most people are more intent on letting FEBuilder write the data for them, but it also tells you exactly what’s going on - if you know where to look for it.
All hacks override previous data for characters. I have yet to see anyone attempt to expand the character table, and for good reason - there’s no value to it. The vanilla table is over 200 entries in length which is more than enough. If you have a project with more characters than that, I would love to see it however.