CM12.1 in Slot 1??... Wipes problems??

Hi!
At last I've got my XT910 running...
But I've got a "little" problem installing CM12.1 Nightly.
When I try to install in "Stock slot", it fails, and my XT910 doesn't start any more....(and I should use RDS lite again to install stock ROM...)
But when I try to install it on Slot 1 (creating a new slot to do it), CM12.1 installs perfectly!!!....
Perhaps it's a "wipe" problem, but I don't knwo how to install CM12 in stock slot...
My steps:
-RDS Lite to install ROM Stock
-iRoot to Root
-Safestrap, and install Recovery
-Here, I don't really know which wipes should I do
-Try to install CM12.1 and it fails, and my XT910 dies....
:(

In slot 1 it works perfectly, but I'm loosing free space....
:(