With it being slate it is to wide a joint for silicone so i would use a schluter one...
here's a link to there expansion joints.......dave p.s. as for ultramastic with slate you will have to ring mapei technical...
Movement and control joint profiles - Schlüter-Systems
i have found this for you aswell might be of use to you.....
http://www.mapei.it/referenze/Multim...stic_TD_EA.pdf