Hmm, that's an interesting take on things. Granted I do not have the current version of the Imperial Guard Codex (the one called Astra Militarum), but I do have the 5th or so edition (paperback) codex. It says that
An Infantry Platoon (note: Infantry and Platoon are both capitalized) consists of a Command Squad... and from 2 to 5 Infantry squads.
Each Platoon counts as a single Troops choice on the Force Organization chart...
As such, I always interpreted that to mean that, no, you may not field a single Infantry squad. In fact, there is no mention of any squad that can be fielded by itself other than Veterans and Armored Fist squads. Since those are both of the "infantry" type, I figured that the formation was intended to give the player a fairly high-quality unit with Veterans combined with the commissar and the tank.