The one advantage of going with the FSB is that they are part of an organization with a chain of command (will have to figure that out for PCs) and common goal, so harder to be a complete lone wolf.
I should mention that as your character is operating with the FSB, his access to explosives would be limited without permission. Sorry, should have mentioned that earlier. C4 and frag grenades would require approval while stun, tear gas, concussion, and similar could be standard kit. The FSB want to take people alive, and limit collateral damage as much as possible. Not to say it will never be available, but just not something he'd be walking around with.
Just a note on interdepartmental relations:
In most movies when Agency A is doing something and Agency B shows up and takes over, that's one thing. They are basically pulling the 'This is our territory, and under our control.' card, and it usually also involves a vanfull of lawyers and an official document stating that they are taking over responsibility for the scene (including any damages, and responsibility for any fatalities or other outcomes of their actions).
When someone from Agency B is on loan to Agency A (which is more like what should be going on in this team) for special purposes, then they are under immediate local control of Agency A unless and until a member of Agency B shows up who outranks their local Agency A controller and pulls the first situation (This is our problem, not yours, and you will obey.). Basically unless that happens, you are there as a consultant under their local control, and nine times out of ten if you call your home office to whine about not having weapons-free, you'll be told to suck it up and soldier, soldier.
There should not be any loose cannon behavior at all.
Additionally, in an urban environment, when SF are sheep-dipped to the FBI (as an example), they will be using equipment that is standard for the office they are loaned to, and will generally attempt to 'fit in' unless they are being loaned as reinforcement to something like a CAT/HRT team (which is a bunch of gonzos in body armor with class III weapons anyway).
So if this is an investigation/find-and-call-in-support type team, those people would be sitting off in the corner next to the SPAT group, because you don't want them walking down the street terrifying civvies (and potentially alerting the targets if they have watchdogs out hidden in the crowds). The advantage to concealable weaponry and sorcery and para-psychic capabilities is that they bring the damage without requiring something that obviously and visibly looks like a fire team.
Admittedly I need to read up on Cascades, but I'd say that is fine. I'm not what (if any) difference there is between a machine pistol and a submachine gun?
It's primarily a concealability factor. MPs are pistol-sized, SMGs are larger (but also more controllable and in many cases more powerful).
Jade, I have a question of my own to ask. I have my own ruling on it, too, but seeing as how this isn't my game, I figured I should ask just the question.
In the books, there's a firearm - the MP6-A1 Machine Pistol. It's a pistol capable of automatic fire. Would the fact that it is A: autofire capable and B: capable of being wielded in one hand allow it to be used with the SMG Gunplay Cascade?
This is explicitly covered in the section on Special Services Training, no house rules needed. It's one of the things which buying SST as an Asset opens for a character. That cascade is otherwise considered rare and specialized. Elite Special Forces (think SEALs not Rangers) and MiBs type stuff.
As easy as spending 4 asset points and taking 1 SAN.
But it would be difficult if not impossible to justify some of the other (hardcore arcanotech) elements.