Note that a ship or squadron coordinated with FFD has the same targeting options as HB when supporting a unit. I've never gotten a confirmation on this, but I strongly believe this means FFD is only effective for front-line formations, since support or rear echelon formations only fire in support of a front-line formation.

I'm also going to note, because I always do, that if your base unit is infantry platoons "companies"    that you are likely to run into a severe shortage of ground unit commanders well before you reach anywhere close to the size of army you need to wage interplanetary warfare (read: multiple million tons). While this isn't a crippling problem, it generally makes more sense to build larger formations (minimum 5,000 tons, recommended 10,000 to 20,000 tons depending on game state and personal taste) to get the most benefit from your commanders. There are also mechanical benefits to larger formations related to breakthrough probabilities, small formations are more vulnerable to breakthroughs against them which can be very damaging. You might consider infantry regiments or brigades with 3,600 UN Soldiers, 600 UN LMGs, 500 Support troopers, and whatever HQ your roleplay deem necessary (actually this might be a little bit too big, but that's hardly a problem...).

My empire is steady growing so its ground forces.

My infantry companies has: 36 UN Soldiers, 6 UN LMGs, 5 Support troopers and 6 commanders.
But now I wants to add 2 Forward Observers, these are in a 2 man detachments assigned to a specific company, then I put a 4 SPG company supporting the said infantry company, will this make the cannons fire in support or has the FFDs have to be a part of a company from the beginning?