I always find it interesting to hear basing ideas - particularly for 28mm fantasy.

Your basing preference will be driven from your previous fantasy rules of choice, and seldom will that be compatible with the popular WRG 6cm elements used by DBA / FoG etc .. and though notionally, if coming from a GW game, the 20mm per fig standard *should* make the idea of creating a large sabot base easy, this does not really cater for the larger 40mm based figures or indeed the simple fact that a lot of the larger 28 models, even GW's own, will fit on a 20mm.

I come from a Fantasy Warriors background originally..where individual figures where based on 1" squares... and so my larger element sized bases are actually 5cm / 2" squares rather than the 40mm used by Warhammer Ancients or 60mm by WRG derivatives. They all have at least 4 figures per base, mostly 5 or 6 but some (my Empire Pikemen) as many as 8.

The cunning plan is to use 2 of these 5cm squares to create a 10cm x 5cm Impetus unit, for both infantry and cavalry. The only potential problem I see at the moment with this is the Skirmish troop type; I have no 1/2 depth bases

I think that base depth in Impetus is not so important. This is mainly for 3 reasons
1) you don't have to retreat or follow up you base depth
2) flank attacks are not so easy to perform and anyway they must start beyond the front line.
3) Interpenetration is very flexible

So Skirmishers in my opinion could stay on a deeper base if required.