I'm not quite sure I understand what you mean by "fusing the boss armor," but I'll keep it simple. For the current boss, if you wish to unlock the PLUS version you need to craft the regular version which becomes available to you once you defeat the boss for the 15th time. Then once, you crafted the regular version you have to level it to 35 which will unlock the ability to craft the PLUS version. You will not have enough material for the PLUS version until you beat the boss for the 43rd time. Once you have however, the next version you do craft will be the plus version.
And yes, the plus armor is a LOT better than the normal boss armour. As you don't have any epic armours at all, I suggest you do work on getting a few EB+'s
For previous boss armors that are fusable such as scorchstone aegis. If you have enough materials to craft it, but haven't unlocked the plus, you CAN fuse for one and level it up to 35 PRIOR to crafting it because as mentioned before, once it hits level 35, your armor smith will craft only the plus version of it.
Hope this helps :)