I've played countless Seleucid campaigns, they are my favorite faction. They do have a very hostile starting location though, which i have always believed is fully intended and probably historically accurate. Seleucid games are ALWAYS hectic at the beginning, but once you get passed that you pretty much steamroll everything in your path. So i would savor this moment in the game, it's one of the few factions that are literally just Total war, personally i love that part. It gives you the opportunity to build up your generals' into fearless killing machines in a short period of time, as well as capitalize on the potential economies of surrounding settlements. It's a very prosperous area.
With my games i always focus on destroying Egypt first as they will get very annoying later on, and i mean from turn one you should have already made preparations to take the nearest settlement. Pull every general out of their cities and start tearing through Egypt until they are gone or don't pose any real threat. Also from turn one its good to send a general into Crete and have him recruit Cretan archers from surrounding area's whenever possible. I usually station a non-family member general there permanently, that will traverse between Crete and Greece every few turns to recruit as many archers as possible. The reason for this is Cretan archers are superb unit and invaluable to your cause. Mixed with Seleucid's already strong unit roster will make your armies extremely lethal.
Also check around Antioch for Elephant mercs early on, they will turn the tide of many battles and enable you to get a foothold in Egypt. Your main goal from here is just to make your land peaceful enough so you can focus attention to the infrastructure of your main cities. Order one of your best general's (preferably faction leader) to govern Antioch and advance in technology and City level as quick as possible. Don't forget that only some settlements are capable of training Elephants so put some though behind your city planning. At this point, the only thing i am concerned about is enabling the training of Cataphract and War-Elephants, as soon as they become available to you, there is really not a force in the world that can stop you.
There is much more to this but much i have forgotten, it's been a very long time since i have played Rome. Good luck with your game and update us with your progress.
EDIT:
Whoops, i forgot about army composition. I usually have 1-2 general's, 4-6 units of Cataphracts, 2 units of Elephant, 4-6 units of Cretan archers, and 4-5 units of Silver shield pikemen. The pikemen don't really see much combat, they are mainly there to deter cavalry charges and to absorb damage from archer fire, most importantly they are there to protect your Cretan's.