A few Q

I don't see why it would be bad to mix LR from different sources. If anything, you have a chance at ending up with a higher level of diversity of hitchhikers (both good and bad). I've mixed LR from different LFS around the town where I live, and while I don't know where each came from, I do know that they are not the same type of LR. The one LFS gets in rocky chunks, while the other gets pieces that are more along the lines of formerly living hard coral (small to medium clustered branching pieces).

"bog standard fluorescent lighting" -- what does this mean? I'm not familiar with the "bog" part. If you're talking about standard fluorescent tube bulbs (T8, T10, T12), then yes it will. I grow caulurpa and chaeto in our refugium using 2 15w T8 bulbs (I think they are 10000K bulbs). The caulurpa loves it more than the chaeto, but both of them grow well. I don't think my bulb choice is ideal, but I was trying to use up the bulbs I already had before ordering some that might be more appropriate (6700K bulbs are commonly used to light refugiums for macro algae growth).

Stocking questions are not my forte, but if it was my 30g I'd go with:
a pair of ocellaris clowns
a pair of firefish (if purchased as an established pair)
sixline wrasse

If you plan on keeping any corals, be advised that while coral beauties are typically considered the least likely of the angels to nip / eat corals, I have seen it happen. Our friend's coral beauty killed off a large portion of his soft corals in his 120g tank. Otherwise, I'm not sure about the size issue and putting one in a 30g tank.

Cardinals are best kept in multiples (3 - 5+).