What do you mean "it won't let you start at that low of xp"? I started Soul Wars on my 99 hp pure when he was level 3 with nothing but 4 woodcutting. Now he's 85 HP and going strong.
Could you elaborate as to what happens?
If you are using the Tip.it calculator for Soul Wars, for some reason it says if you are below 20 hp that it is to low to start. That is incorrect, you can start at any level.