As long as all of your models are based consistently, although they may not be ‘tournament legal,’ they will work perfectly well with the Swordpoint rules. Ideally, the bases/sabots for your Skirmishers should have twice the frontage of your formed units, and the bases for single-based models should be scaled proportionately. For example...
Formed Infantry: 6cm x 6cm (should easily fit 4 bases of 25mm round-based Saga models)
Foot Skirmishers: 12cm x 6cm
Formed Cavalry & Camelry: 6cm x 6cm (7.5 cm x 7.5cm if you want to keep the proportions from the rules)
Mounted Skirmishers: 12cm x 6cm (15cm x 7.5 cm if you want to keep the proportions from the rules)
Light Chariots: 6cm x depth to suit model
Heavy Chariots: 8cm x depth to suit model
Elephants & Scythed Chariots: 8cm x depth to suit model
Wagons: 12cm x 8cm
Commanders: as per the rulebook
That’s just one possibility. There are many creative solutions possible, if you want to play Swordpoint with your Saga-based figures.
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