Well, speaking from my own perspective:
1.
Generally, if I was squad leading I wouldn't want to pull Prowlers unless the next part of my plan involved mass Armour/say moving out on Esamir from the Warpgate.
Practically speaking Prowlers are both quite powerful, and quite weak - as a battle tank they're probably the worst of the three as a base line unit as they exist in game at the moment. They're huge targets, their guns are awkward to use at range and their special ability is pretty rubbish.
They also come with a hefty respawn timer and are significantly less maneuverable than Sunderers. Remember that our reason for being in the party bus is mostly as it being the most efficient way to get the platoon between targets, and having a Prowler along would force the column to slow down, especially when we try to go off road.
The Prowler definitely has it's place, but I think the cool down is better saved for dedicated Tank column or as a special weapon for use when breaking into a heavily contested courtyard.
2
As far as I'm aware there aren't any formal guidelines. I've seen a few guides here and there, but it's not like Planetside 1 where we had an "Everyone is an advanced medic" rule. I think we're still mostly learning the game and trying to figure out our role in the grand scheme of things.
From experience, I'd say the key is to be flexible - the basic version of every class is for the most part capable of performing it's role in a squad. Learn how to do multiple roles, and don't be afraid to swap jobs on the fly if you notice the squad is seriously lacking in something. The Platoon Leader is generally too busy to worry about the minutiae of the squad makeup, so anything you can do to make their lives easier is a good idea.
3
Again with point 2, I don't think there's any priority certs perse with the possibly exception of AMS for Sunderer. They simply make the Sunderer as troop transport so much more valuable, and the more people with that cert, the less likleyhood that, as a team we will ever be without one when moving into battle.
Still, at the risk of undermining what I said in Part 2, I think it may be wise to develop your own area of specialization. For example, when I look at the platoon lists I see the same names typically playing the same roles - for example Hindu is almost always playing Infiltrator which covers the platoon need for a hacker. Pain usually seems to play Heavy Assault. Doogle and myself usually seem to be among the first to pull Maxes whenever the call goes out for them.
Hence I think it might be an idea, though you might want to check with the officers, that becoming "The Man with the Mines" might be useful to the squadleaders at least in terms of them knowing at a glance what options they have available. Again, we won't always need tank mines, but even in Planetside 1 where we all had advanced medic at our core, we had our usual Max operators, Gal Pilots and others. Familiarity isn't a bad thing.
Hope that helps.