GW encourages the idea and I'm fairly certain this was always the case. People did it in Dawn of War see:
https://www.dawnofwar.com/article/army-customization-spotlight and then there's the mods too. Not sure how many other
WH games feature customization, but go for it.
It is one of my greatest attractions to the verse in the first place. Plus as I understand it (not a competitive player) no one cares about your lore as long your army plays by the codices.
Youtube is a great jump point for "how to", if you search up creating custom chapters
40k or something of that order.
I would say Iron Hands should be a great first founding chapter to start with. They are calculated heavy hitters with a reverence for their war engines which they maintain and pass down in tradition. They have several successors with which you can splinter off your own from or from the Iron Hands themselves.
The Iron Hands codex supplement will give you some back story on their lore, their chapter organization which is maintained by the Medusan clan companies, their strategic make up, special characters, what their homeworld is like, how they recruit, a name generator, stratagems and tactics.
If you go with Iron Hands as a direct successor then you inherit their chapter tactic, same if you chose one of their successor chapters, you inherit it from them. You are all Sons of the Gorgon.