Some work sir!
Chris, did you cut off the tabs or build up the bases?
I glued them directly to slotta-style round bases. Most are on 25mm, the chieftain and war dog handler on 30mm, and the war dogs on 40mm, although I considered the small oval bases by GW for the latter (they looked a bit off, in the end).
The rest is PVA and a sand-grit mix. No putty or something, except for some larger stones placed to slightly conceal very obvious steps (some of the integral bases seem to be a bit thicker than others).
Conversions and both painted examples very inspiring. 
Mine are still in the bag. 
That said, I've put them with my few remaining Celtos and a lot of Renegade Miniatures Celts I have recently acquired with a view to have dense crowds of multi-based figures. Still debating which base sizes to go for.
It's tricky. Most of the figures should fit on 25mm washers (i.e. lacking the bevel of the slottas), but some have a rather wide stance with quite some overlap. I must admit I'd be unconvinced about using multibases for these since it would probably reinforce the similarity of the figures' poses.
