So Simon has a video that covers this very topic and gives Impetus examples.
I did decide to convert my Impetus bases to MeG (can still use for Impetus - just uses more figures now though). Though I really didn't realize what an absolute pain that is!
For 28mm I think Impetus works best for PACTO using two bases to represent a 2 base unit. That way you can mark one wound and remove a base with a skull. You need a pretty darn big table as the BW is now 120mm (double) - say an 8x6 table.
[I bought a mousepad map 6x3 and cut a 2x3 for my 15mm PACTO and a 4x3 for my 28mm PACTO and also have a separate 6x4 for my 28mm Magna as rebasing made this more flexible]
For 28mm MAGNA I would consider using 1 Impetus base as a four base unit. The only difficulty is that as you remove rear ranks with the first two skulls you won't be able to adjust the depth. You could use a skirmish base if you put the correct amount of figures on it I suppose, but not useful then for Impetus. So a 6 unit pike block would be 1 Impetus base plus an Impetus skirmish base in the rear.
Maybe someone has a better suggestion for Magna and Maximus?