I would think, being pelargoniums, that you could prune away quite a lot , be quite heavy handed. This works with my pels.I prune hard each autumn and reduce watering throughout the winter,but it could be done anytime of the year really. Some plants react badly to pruning but in general if you want something to grow prune it hard!

(This doent work too well with my Pelargonium dichondraefolium, which is a slower growing lavender scented plant, but most scenteds are quite vigorous growers and would take it I'd imagine)

Plus you'll have plenty of cuttings as backups!