The Legionnaire is bigger than the Terrapin, but not so big as a Vanguard. Pad size really doesn’t help when you’re looking at big deck spaces like the Idris. I think the Idris will be able to squeeze Legionnaires a bit closer than folks might guess. Also, though I haven’t checked, I’m pretty sure the Legionnaire will fit the Polaris bay, and the Vanguard will not. So there may be big utility there.
There is a good vid on YouTube asking the question whether the Legionnaire isn’t a cleverly designed multi. There were some decent points made but the range issue went unexplored. If CIG tweeks range on release, there are a host of possibilities. Also the reactor signature. . .if it’s as low as we might expect from a thick hull ship, that would matter too. So don’t count it out. A pair of them can’t match the firepower or speed of the Prowler, but they may have other uses given lots of armor. It really depends most on just what the hacking suite is all about. That’s the unique feature they haven’t yet explained. What if like in ffg, you can tell another ship to power down engines, shields or reactor? There are a lot of ways CIG could go.