Yup, spike shoes don't need to be from the same brand - it's effectively a piece of metal with a non-slip rubber bottom and a whole in the top where the spike can sit.
I should say, the Nexus 6 stands were given 4 stars in the latest issue, not 5 as I said (must not have had my eyes in when I skimmed the issue!). They're still good, though the offer you've got on the Soundstyle's sounds good. I haven't heard the sub you mention, but if it's as good as the Mk I version, I think it'll be fine. If you think it sounded great though, I'd say that's the most important detail!!