I've only progressed up to around 220 B.C. as the Seleukids, M/M. VH campaign difficulty (which is recommended) will change things. And, I haven't played a Seleukid campagn on .74 yet. (Some things are different, now being able to recruit Pahlava Shivatir, for instance.)

The Seleukids have a nice line up of troops, typical successor army troops, pezhetairoi, pantodapoi phalangitai, hetairoi. You also get eastern archers, and Mad Asbara; decent medium cav. Word is, if you manage to get thoroughly trounced by a Hayasdan or Phalava army with Kataphraktoi, your general may get the "Intrigued by Kataphraktoi" trait, and eventually start training Hellenistic Kataphraktoi. If not, well, you'll just have to make do with Hetairoi and Mad Asbara.

Your capital is Antiocheia. If I'm not mistaken. Seleukeia starts out as your biggest city, but Susa, Persepolis, Ekbatana, and er, Gabai sprout up pretty quickly, too. Happiness will plummet in the cities in the East, so you have to decide if you're gonna hang onto them or clear out, pretty quick. A hint, you'll wanna hang onto Sardis and Ipsos in the West; if you can build up some mines (and acquire the Miner or Superior Miner trait), they'll start pulling in some decent cash.

So, long story short, yeah, it's a challenge to play them. Obviously, you're gonna wanna get some paved roads down ASAP so you can haul ass here, there, and everywhere. Paved Roads give +5% Happiness (due to Order?), which is nice.

You're gonna wanna get your economy together in a hurry; the Seleukids are rich, but not Ptolemaioi rich, and they're gonna come up with Kleruchoi Agemata and Galatian Agemata in serious numbers. Pahlava will come at you, too, but they can't recruit any better than pantodapoi phalangitai, so as long as you can recruit pezhetairoi, you should be OK.

I haven't even tried to fight the Pahlava in the open field, so I can't offer any tips there. Bring lots of archers and hope for the best? ;)