Basing for newb

This question gets asked pretty regularly. Base size matters.

For some reason the culture around GW games can be extremely nitpicky about such details. The most common base sizes for KT figures are 25, 28 and 32mm. I'd be absolutely fine with 30mm base being substituted for any of those. Like others have said, official tournaments might require you to use the exact base sizes, but otherwise it shouldn't make a difference.

My advice is: If you are planning to attend official events (vast majority of players dont), go with the bases provided with the minis. Otherwise do what you want.

The operatives data cards tell you how big their bases are meant to be. There are advantages/disadvantage to proxying models with different sized bases.

Check with whoever you are planning on playing against, if they are casual players who are more interested in the social side they may not care too much. You’ll struggle if you are planning on entering tournaments as you may get accused of modelling for advantage (assuming the tournament even allows proxies)

Yeah, base size is used for navigating terrain, and for determining line of sight and cover. It's really important, so you need to stick to the base size that comes provided with the miniatures. Unfortunately there aren't any that use 30mm. The standard base sizes for most infantry models are 25mm, 28.5mm, and 32mm.

As others have said, base size is important and can in certain cases be both an advantage and disadvantage.

I've seen people want to keep a universal base size across their collections, but still want to play the game. These people usually just blue tack (or magnetise) their models bases to the correct size base (so in your case, you'd magnetise a 30mm base to a 32mm base) when they are playing, and then remove them for display. You can try that. This obviously doesn't work when going to a smaller base size.