I am creating an Alchemist in Pathfinder and want to take the Tentacle Discovery - This grants my character an extra "arm" in the form of a tentacle.  My DM told me that, with this extra "arm", any armor that drops will not fit me.  

What are some Pathfinder rules that will let me work around this?  

I was thinking of getting Artisan Tools and the Craft Armor Skill, but I do not know how effective that will be for modifying existing magical armor.  

Any ideas?