All BBs but Andrei Pervozvanny and Nelson can hold scouts. As for Engies and reps the common setup is 5 engies 3 reps, or you could run 6 engies 4 reps if you don't use T slot gunners.

i thought the common was 4/4??

From my knowledge more people run 5/3.

I thought engys and reps don't work unless they are in the support slot?

Reps work on all slots, Eng only on support slots.
You will need to overvet 4 reps for the repair cap and seriously overvet them if you are planning with 3.

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5/3 is good with AA or6/4 without AA. You don't really need to super vet your reps, just give them 100 vets and boost, hitting a rep cap does not mean you dont get 1 shot from A monty, so no reason to spend that extra money.
If you were starting SN crew, you need very good engs.

depends on the nation. KM you use mainly 3rep 5 eng, other nations you could use 4/4. I use mainly 4 and 4 because well lack of range bb's gunna get hit more so i'd like to repair faster with the other nations. But with all nations get either Elites 12's or if you have too 11's. 10's aren't what you call a good thing anymore. It takes 10's a while to cap out and until you cap em, they just don't compete with the other stats.