Yes, there is a difference, so I would pick one and stick with it.
As said at the moment 10mm has the greater range for WWII and modern.
For small actions, say a reinforced company, a 4’ x 3’ gaming area is more than adequate.
As for rules, use any. If they are based in inches, convert to centimetres. I’ve never felt that 10mm / 12mm looks particularly good for single based figures, so I would go for stands / groups, even if playing something that usually advocates single basing. A ton of people are quite happy with 10mm on singles and will disagree with what I have just said – as with most things, most of this is personal preference.
You could probably do with putting in a sample order so that you can physically see what is what.