Well infiltrators need to be at least 18" away from another squad (including infiltrators) or 12" away if they are deployed in terrain, so you wouldn't be able to infiltrate around them as you say. However, I see no reason why an infiltrator squad with a seveyor couldn't open fire on them still.
I beleive the attack is resolved after deployment, so it wouldn't make any difference being an infiltrator yourself, but remeber, it is only 4D6" range, and you will be 12" away minimum - but you should be able to get it, just don't depend entirley on counter-infiltration.
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