Here are a couple of examples I happen to have been working on...

A lot of the Bloody ECW / TYW figures are perfectly suitable for the earlier period, especially the figures wearing tabbed doublets. There are a fair few of these sprinkled through the range. All the figures in 'The Ragged Trayned Band' for instance - except for the one in the characteristic ECW montero, who would require a head swap.
You could probably also get away with using all the figures in the short 'soldier's coat', which was a shortish uniform jacket - a slightly longer but much simplified version of the doublet, and a step along the road to the long, more enveloping Dutch or long soldier's coat, which morphed into the uniformed frock coat which clothed armies from the 1660s through to around the 1770s, when turnback tails, short coatees, and other cut back forms started to take over.
But in terms of the transition from the 1610s to the 1640s, I doubt there was a hard stop where doublets suddenly gave way to jackets. Fashions evolve over time. Nick, the sculptor, has given most of the new Jacobean era figures stand-up collars or 'whisks' and long boot hose tied up with garter wraps, just as a way of differentiating the figures somewhat from the slightly later period. But these styles would not have been worn by everyone, high to low. Basic breeches, stockings and latchet shoes (or boots) would have been worn by many people.
Buff coats were certainly around in the 1600 - 1620 period. It's just that they have a distinctively (iconic) ECW look because they're so closely associated with that conflict and era.
The cassock or 'casaque' - which could be worn as a cloak, tabard or riding coat, with sleeves on or off, all depending on how one buttoned it - appears to have been fashionable from the 1630's onwards, so probably just a wee bit too late for the Jacobean period. But I doubt anyone is able to say for sure.
Anyway, here are some examples...
One of the 'Squire's Men' with his antique fowling piece and tabbed doublet, with a morion head swapped onto his body, and with the addition of a Green Stuff plume.
And a couple of the armoured pikemen, equipped with the tufted partizans from 'The Bluff Soldiers' set, and with added plumes.

A fairly quick and easy way of filling out one's force of Yeomen of the Guard / Gentlemen Pensioners... I think they all fit together reasonably well?


Next up, I've used one of the 'Bloody Practitioners' (gunners and engineers) as an all purpose thug or houndmaster, putting one of the cudgels from 'The Bloody Hounds' weapons sprue into his open hands.



And here, just for comparison, are two of the same houndmaster figure - one painted as a soldier armed with a halberd, the other as a civvie armed with a cudgel.

I've got a few more on the go, which I'll share once finished.