2 questions on MLP preferred stock and common units in a tax advantaged account

[MLP_Banana]

Hello,

Do "guaranteed payment for use of capital" (box 4a on a K-1) distributions reduced the cost basis of MLP preferreds, in the way that distributions reduce the cost basis of common units?

Also, I have some MLP common units and some preferred units of the same MLP in a tax advantaged account (a Roth IRA).  The common MLP units have appreciated quite a lot.  If I sell them in the Roth the capital gain  on the sale may be classified as UBTI, and incur a tax charge in the Roth. 

Does anyone know whether I can sell preferreds on the same MLP at a capital loss, and use the preferred MLP stock  capital loss to offset the common MLP unit capital gain, thereby perhaps reducing UBTI on the sale of the common units?

Thanks!