I would say –
15mm as a cheaper alternative to 28mm for big battles (sounds like you want to do this rather than skirmishes) is a good idea. You don't lose too much detail. Huge, cheap ranges for your period are available from Essex and many others. I've got a half finished 15mm ECW unit from their range so can recommend.
10mm – there isn't much around for this period in this scale. I've never bought anything in 10mm so can't say much more than this.
6mm – I'm currently collecting for this period in this scale, and between Irregular and Heroics & Ros, you've got everything covered with plenty of variations.
What you can do with 6mm is, if you want, 1:1 ratio of figures on table to real soldiers within reasonable space and money limits.
What you can also do is use 6mm figures for different ratios – 1:100, 1:200, 1:300 etc, without losing too much visual effect, because the eye rests on groups rather than individuals. So basically you can get a big battle on the go quickly.
That said, both the manufacturers I mentioned above produce detailed enough figures that you can see different weapons, arms, legs, heads, different helmets and hats etc so the period detail still comes across (especially when the costume at this time includes hose in one colour, doublet in another, halving and quartering etc). Easier to paint than you'd think (base colour, wash, pick out details).
Worth knowing that Irregular minis usually come posed on one strip (pikes squashed together) whereas H&R give you more freedom for arranging them.
There's also 2mm, where you're basically getting a block shape where you can just barely make out the individual figures. Apart from that, has the same virtues as 6mm but more so.
For Dutch vs Spanish background reading, there's a very cheap and exciting (albeit dated and biased) biography 'William the Silent' currently available on Amazon, which doesn't give you total detail on the battles, but definitely brings across the atmosphere and personalities.
Rules that support 6mm models – DBR, Impetus and probably several more but definitely these two.