To clerify, it doesn't really make much of a difference how many souls your character's starting ability or starting abilities cost, you can use them as long as you have enough souls. And, since you can start off with a maximum of 15 souls, how many souls it takes to get any ability you start out with can be anywhere between 1 and 15.
Like if you started out with 12 souls and you wanted to start out with an ability that fired lazer beams. It doesn't make a difference wether it takes 1 soul to unlock it or 12 souls since, either way, your character will start off with the ability. It's not really necessary for abilities to power up at 5 souls or for them to take 5 souls to make the ability, but it is a good kind-of guideline. You know? As long as the bio isn't overpowered, makes sense, and fits the world, it'll be accepted.
Anyways, when I said 20 souls per ability, that was as an example. There are no real references for how long it takes to unlock each ability, but assuming you get a new ability at 20 souls each time, wich is well within reason, you'd have 10 full abilities at 200 souls and 15 full abilities at 300. Which would already be an INSANE ammount of power! So, in my honest oppinion, at somewhere between 200-300 souls, your character might as well be considered a full-powered Kishin. They would definatly be strong enough for it at that point.
Worst case scenario, you should be able to have the title of full Kishin at 400 souls, since then you would probably have between 15 and 20 abilities, depending on if you're going for 20 or 30 souls per ability on average. And, well, 20 abilities is definatly enough to warrent OP status.
Hope that clears up some confusion.