Let me start by saying, cool! I really like the look of the Haradrim force. I think the main weakness of the Fallen Realms is they have relatively few Epic Heroes, and only one caster- Queen Beruthiel of Rhun- so you will probably want to take some Mordor allies in order to include Ringwraiths or such gentry.
Infantry comes in companies of 8, cavalry in companies of 2. Formations are made of companies. There are three types of formations: common, rare, and legendary. You may not have more rare or legendary formations than you have common, and you may not have more companies of rare or legendary than you have of common. You may only have one of each legendary formation.
Your common are:
Haradrim warband, 25 pts per company, a formation can be up to 9 companies strong. They have hand weapons and poison. The entire formation may be given bows at +5 pts per company.
The command company can be upgraded with chieftans, banners, hornblowers, taskmasters, and a Hasharin.
Haradrim Raiders @25 pts per company, up to 6 companies in a formation; again poison, and again may be given bows at +5 pts/company.
Serpent Guard, and Serpent Riders, are 30 pts per company, one to six companies for either. Poison. They have slightly better fight and courage. Riders can be upgraded with bows.
For rare, you can have
Watcher Warbands @ 35 pts, up to four companies; ambushers & poison. Can be given bows @ +5 pts/company.
Half Troll Warband @ 95 pts/co, up to 3 companies. Berserk, Indomitable, Terror.
Mahud Warband @30 pts/co, up to 6 companies; can be given blowpipes @ +5 pts/co.
Mahud Raider Warband (camel riders) @30 pts/co, up to 6 companies. Again, blowpipes @+5 pts/co.
War Mumak @ 250 pts ea. One Mumak and 13 crew... .
Legendary Formation:
Abrakhan Guard @100 pts + 30 pts/each addl. co. (and the pictures in the book look like plastic Haradrim Warriors with a different paint scheme).
Epic Hero:
Suladan the Serpent Lord @ 150 pts. Amongst his special rules, the first time each turn he expends might, on a
d6 roll of 4+ he regains a single might point. Tasty.
That's just the Haradrim, mind. The Fallen Realms also include Corsairs, Easterlings, the Khandish Kingdoms, and the Black Numenoreans. And then up to a quarter of your points can come from an allied list (Mordor?).
So: assuming you bought plastic Haradrim, right now you probably have 4 companies of hand weapons and 2 companies of bows. You've a company and a half of Serpent Guard, 3 companies of Raiders, and two and a half companies of Serpent Riders. The chieftain and banner can go into the appropriate company- maybe use them to fill out the Serpent Guard. You can put together what you've got, to give you five common formations. How many mumakil did you say you wanted?
I think an army of about 1500 to 1600 points is a good starting size, and your troops aren't terribly expensive.
I'd probably invest in another box of Serpent Riders, and enough Serpent Guard (including command) to make three full companies. I'd buy two mumakil, another box of Haradrim horse, and at least one more box of plastic Haradrim warriors- more likely three. (A box basically gives you 2 companies with hand weapons and 1 company with bows.)
After that I'd start looking at other things in the list, that I can get cheaply in plastic. Corsairs? Easterlings? Allies from Mordor? Your bog-standard Mordor Orcs are 15 pts/co.
Only then would I start looking at the more expensive metal units like Morgul Knights, Half Trolls, or Mahud Raiders.
Who are all these people, and why aren't they dead? 
