A lot of people (probably very wisely) do recommend only collecting one scale, so you save money on terrain, accessories and the like. This makes sense.
However, there are a few genres which I reckon lend themselves to skirmish stuff (ideal for 28mm) and some genres which lend themselves to big battalions (little stuff - your wee 10mm fellas).
Despite my feeble attempts to justify a smattering of 95th and 60th rifles, plus a token battalion of line infantry, in 28mm I would go for a smaller scale. I do have boxes and boxes of 1:72 Napoleonic plastics just in case I am trapped in my studio by a freak landslide (one that goes uphill) and have to spend a month or so doing nothing but painting.