The only one I would rule out is the FAL. I'm a FAL fan but with a 13" or worse an 11" barrel I don't see the point of having a .308. I'd rather have a cheaper round that is more efficient out of a shorter barrel. Even 16" 308s have a lot of blast and flash.
Of the other three, unless money is no object and doesn't factor in at all, I don't think the SCAR is really worth $3K. What does it do $1K+ better than the others? Between the sig and the XCR, on paper I like the features of the XCR more. Its adjustable gas system is also very nice if you intended to shoot it suppressed. Both the USA sigs and the XCR had issues early. The best I can tell both companies seemed to have worked through that. Rob arms customer service was known to be very bad initially. I have read that major strides were made in that respect. I'd lean towards the XCR and the SCAR and probably go with the XCR as that SBR scar seems a bit overpriced IMHO.
Other Options:
I like draco and mini draco SBRs. The mini with a 7.5" barrel is rather compact. However, the OP states he wants to suppress it. One certainly can put a suppressor on either of those guns. I intend to do just that. However, they are not ideal suppressor hosts and are not rendered as quite as other designs.
Mini dracos also seem to be in short supply right now. This means the command a premium. Furthmore, if AKs are being considered the option of the PAP M92 PV Pistol with a 10" barrel should soon be available soon as well and deserves a look. A mini is only 16.5" long. However a folding mechanism will add and a little length, and it really needs a flash hider (the mini is a flame thrower) so depending on what you add (I'd recommend a vortex) that will add some length. Still it will be very short when folded, less than 20" I'd imagine. Furthermore, the x39 round does really well out of a short barrel. I don't recall the exact numbers but I want to say that it was doing about 2000 FPS over my chrono.
Another 7.62x39 option would be the PTR-32. After getting your stamp, you should be able to pull the end cap and add either a telescoping stock or a folder. The PTR takes AK mags, has an 8" or so barrel. Basically it is a slightly larger package than a MP5 but with a harder hitting 7.62x39 round.
You mention wanting something extremely small. I'm not sure what your intended uses
An SBR FN PS90 is smaller than all of your options. The PS90 SBR is 19.7". The short FAL is 34.5" unfolded and 25.25" folded, the Sig is 29.8 unfolded and 19.7" folded. The XCR mini is 20" folded.
I'm not sure of current availability, but I have seen SBR AUGs with a 14" barrel. For 5.56 you don't have massive velocity loss you get with a 10.5" gun yest you have a gun that is going to be (I'm making an educated guess) about 25" long.
