. If anybody know something about the ancestors of the Sklavenoi, please write it

I think they were good archers. Really though, it depends on the area of the Slavic tribes. A suggestion for units would be light spearmen that have a good offense. I found salvagable ideas from various articles.

Slav Warriors
These ancient warriors are described as being unusually tall and strong, Slavic warriors typically used a wide range of weapons. A single warrior may carry a shield, spear, ax, sling and bow.

Slavs expanded they began using combined arms tactics, with archers, cavalry and infantry working in unison. Their armies used speed to their advantage and often employed ambushes, flanking assaults and guerilla tactics

In their dealings with Sarmatians and Huns, it is not inconceivable that the Slavs became skilled horsemen, a feature which might explain their successful expansion.[101]

Spears/javelins and "nice" shields are mentioned as characteristic Slavic weapons even by Arab authors in 9-10th century so we might assume it was most typical weaponry of Slavic warrior.

The axe was a favourite weapon among the tribal warriors. Cheap but devastating, the Slavs made good use of domestic axes (topor) and specialized battle-axes (bordy) of varying weight and blade shape

Being the finest warriors at the disposal of the knez, they are the military elite of Slavic society, and their armour and equipment is evident of this. Not until the 9th c. that armour and swords had become more widespread among the Slavs and till that time only the Druzinniks had disposal to such fine weaponry.

The absence of armour was a common feature among the early Slavs. Not till later periods (9th c.) during which clan chiefs were consolidating their power do we see wider use of swords, helms, and chain mail.

http://www.twcenter.net/forums/showthread.php?218736-Kry-i-%9E%26%23283%3Bl%26%23283%3Bzo-Early-Slavic-Warfare

http://www.ancientmilitary.com/ancient-slavs.htm

http://en.wikipedia.org/wiki/Early_Slavs

Hope my suggestion helps.