i would suggest keep playing whether its normal or hard.  AI is somewhat random; sometimes they play well sometimes they are horrible.  You might find one game easy and other times hard.  Its good to play normal till you can beat Hard.  To simplify things, only use one opponent.  The more AI you have the more random factors there are.

The main difference between normal and hard is that hard cheats.  They still have the same basic idiocy.

One of the AI's most notorious tendencies is to build lots of light frigates (cobalts, disciples, skirmishers).  These units require no research, but are destroyed very easily by long range frigates (javelis, illuminators, assailants).  If you can upgrade to those unit types at the start of the game, you can kill armies many times larger than your own.

Another stupidity of the AI is the fact that it picks capital ships at random.  This is extremely stupid, since for most factions there is an obvious best choice for first capital ship.  For Advent and Vasari, you just aren't playing seriously if you don't pick the mothership or evacuator.  Oh, sure, there are a couple non-standard strategies out there that use different capital ships, but for a balanced strategy these should always be your first choice.  For TEC, it's usually Akkan or Marza, although occasionally you see Sova or Kol.

There is also the fact whether the Hard will go either be an economist - which is very easy, fortifer - medium, diapolmat - easy, and aggressor - hard, that will represent how hard or how many ships that they have and how many they hit with. So you think 1 AI on hard is hard eh, wait till you get 9 unfair aggressor on you, all on the same team...

Each AI has its various strengths and weaknesses.  Aggressors can actually build huge armadas quite early on, but they're all unupgraded siege frigates and light frigates.  If you survive their initial onslaught your capital ships will be such a high level that the enemy will pose no further threat.  An aggressor grossly mismanages its economy and nearly ignores research.  An aggressor AI is dangerous for about the first 30 minutes, and a contendor for the first hour.  After an hour, aggressors usually are so far behind economy and tech wise that they're powerless. 

Fortifiers suffer a similar problem in that they fill every tactical slot in their homeworld before they start to expand outward, a huge waste of cash. Unlike the aggressor, however, this does not mangle their economy long-run and if left alone long enough they can be a significant challenge.  The best way to beat a fortifier is to rush him.  His fleet will be very weak from all the money he's spent on those turrets, and so long as you fight outside of their range you can easily win.

Researchers are definitely the hardest.  The AI's biggest flaw (even on researcher setting!) is that it doesn't progress through the tech tree fast enough.  These guys actually bring out units soon enough to be effective, and their fleets will progress at a reasonable pace.  They're sensible about defense and fleet spending, build a reasonable economy, and are definitely the most balanced oponent you will face.

Occasionally, you may notice a particularly weak AI had lots of civic research, but no military research.  This is the economist.  Like all AI's, he cripples his economy by getting too much fleet research.  The complete opposite of the researcher, he largely ignores the military research tree causing his units to be laughably weak.  This makes the economist a very weak opponent.

Nope; try playing a game without buying any fleet upgrades (you can easily survive just by picking a Marza and power-levelling it).  The AI will happily advance through the fleet upgrade levels regardless of the fact that you are not.  I often will stay at one fleet level upgrade against the AI throughout the early game, since it's easy to have higher level units than they have and crush them.

Maybe I'm just a bit slower on the fleet upgrades than I should be against the AI, but I almost always have lower fleet research than the AI until the late game when my economy starts to really take off and their's is just left floundering.

quick and shameless cheat.  Play the first 20 minutes with all the AI's on easy/normal.  Then, once you have a bit of a fleet, an okay economy, some depth, and some very heavily fortified chokepoints, save your game.  Exit and reload the game.  In the options screen, set the AI to the level that you really want it to be at like hard/unfair.  Now that you have a bit of a setup, have fun playing with one empire against the other. 

Also, figure out where the enemy fleets will be coming in. Seed it with mines, and place a starbase with fully upgraded weapons, armor, and hangers,(and if TEC, at least one level of self-destruct).  Watch their fleets run straight into your toughest defenses, and then be annihalated when the starbase blows up.  Support with flak frigates(starbases are weak against fighters), and Long-range LRM's.  See if you can get a couple of repair platforms in support.  Watch a small fleet destroy much bigger fleets

Anyone who's going to complain about me cheating, read the first sentence.