The Greeks did use the phalanx, but not the Romans, not at least during the time of Caesar and on, by the time they developed the Legions as we know it they tended to vary their formations as needed depending on the opponent and the terrain.
As for the pilum, to the best of my knowledge it wasn't barbed, it instead had a relatively soft metal shaft behind the head that would bend when stuck in an opponent's shield after being thrown. The reason for the bending was so that it wouldn't be so easy to pull out of the shield and the person bearing the shield would either have to spend some time and effort to pull the pilum out or simply ditch their shield, either way making them more vulnerable to follow on volleys. I've also read that a secondary purpose was so that they couldn't be thrown back at the Romans if they did get pulled out since, apparently, the shafts couldn't be straightened back again without a blacksmith and a forge.
In regards to decimation, you got the definition partially right, it was a punishment for a unit that ran in battle in where every 10th man would be put to death by his fellow Legionnaires.
Most ancient armies used some kind of close formation. Probably discovered in very archaic times by those guys who would stick around the huge guy with the massive club nobody dared attack, they figured that if you have a buddy on either side a few behind you, Your task assigned in a combat situation becomes that much easier, just stab the guy in front of you rather than develop eyes on your back and avoid being stabbed in the back in the middle of a mob.
Now the Greeks perfected the system by having heavily armoured soldiers in full armour with a wonking big shield and a spear ramming into each other. It is believed that most ancient battles would have some kind of stand off distance so you could throw spears, javelins and curses at each other. Fighting may have been done on a more local level with groups surging against each other before falling back to catch a breath, sort out the wounded and bring up more men from the rear. Because the Greeks were so well equipped they went for the Othismos, often translated as a "push". Some think it was akin to a rugby scrum with both sides simply locking shields and pushing until one side gave way, others believe that such pushing would crush and suffocate people and that othismos-push was more of a figure of speech not to be taken literally.
Standard Greek phalanx formations were usually 8 man deep, but the Macedonians turned it into a steamroller by making it much deeper and giving each soldier a long pike five meters or longer in length, which presented a wall of spears to any who tried to take it head on.
Phalanx formations were very powerful, but they were unwieldy as it required troops to keep aligned to preserve the protection of the locked shields, hard to do on broken terrain or with a bunch of dead and wounded solders and a lot of sharp spears and swords on the ground underneath you.
The Romans also used the Phalanx in the early days, but they developed a different system of three lines divided in smaller units called maniples that operated independently and no longer relied on linking up with the rest of the army to be effective. The Romans even left gaps in the line so they could manoeuvre their troops back and forth if required and either allow you to fall back and reform the line if the enemy was strong or allow the second and third lines to join in the fun if the enemy started to weaken. Instead of the spear the Romans used the pilum, a special javelin that had a long thin barbed head that would pierce shields and armour, once through the head would bend and the barb made it hard to pull out. If it got stuck in your shield it would imbalance it, making it useless and the bent head made it nearly impossible to pick up and throw back at the enemy. A few thousand Roman soldiers throwing a pilum at an enemy force would disrupt them before the clash, where the Romans used a short stabbing sword, ideal for close quarter combat. If the phalanx was a pincushion, the Roman legion was a buzzsaw ...
