I bought Strahd's Possession and would like to start playing. I know that I can only make two characters, can anyone suggest useful classes for this game?

Pretty much what stryx said.
You create two characters and can add two NPCs.
You need a cleric and if you plan to transfer your party to Stone Prophet, you need a mage as well.

In the first game, you can find Mage NPC pretty quickly but no cleric until a ways into the game.
In the second game, there a NO Mage NPC.

Also, be aware of level-drain, quite a number of enemies can drain you and there is no way to restore lost levels.
So Negative Plane Protection is essential.

Bye.
Blackboard Monitor

There is one way to undo the level drain. UGE - Universal Game Editor - enables you to alter the xp for any class of any PC. Not only the xp but also the level(s) too. Ditto the attributes of the level: Thac0, spell use and so on...
For details see my new topic all about UGE.

Thanks for the information and advice, everyone!

My party line-ups were:
Ravenloft 1: F/MU, C/MU, Fhalken (F), Traned*
Ravenloft 2: F/MU, C/MU, Trajan Khet (R/C), Hrak Tur (F).

* Cleric/Ranger version of Tatian (F), created with the use of UGE

I also found this mini-FAQ https://gamefaqs.gamespot.com/boards/564797-ravenloft-strahds-possession/75513633
It says that NPCs are useless and only leach your experience points :-/ Is that true though?

Ehh, I wouldn't agree. While it's true that having NPCs in the party means that your XP is divided across more people, depending on the makeup of your original party, they can help fill roles that you're lacking and provide additional firepower. Some people do like running a minimal party with twinked out PCs, but in my experience, having more spellcasters along for healing and spell variety helps a LOT in the SSI games, especially in Strahd's Possession where there are plenty of level-draining undead and you do NOT want to engage in melee with them unless you absolutely have to.

Additional NPCs are useful, at least as mules. Additional spells are also useful and having a thief is useful too, as there are some locks, that can be picked and it streamlines gameplay, if you don't have to hunt for keys.

A thief is especially useful in the second game, as almost any lock can be picked and you don't really need keys at all.

Even with additional NPCs your starting characters will gain enough xp to easily finish the game. There really is no need to jump through any hoops to get additional xp.

Yeah, NPC are effective as mules indeed!
PS - do i need Prayer Cleric Symbol (or Paladin's)? I thought that clerics already had "builtin" cleric symbol to cast spells lol

If you just found that symbol, hang on to it. Probably a quest item, can't remember.

Ok, i just found not 1 but 3 same cleric symbols, i thought they were disposable items..

The quest item looks different. The generic cleric and paladin holy symbols are useless.

Things that look like the attachment can be thrown away.