Ian Heath (Armies of the Dark Ages), says:
“After their conquest of Hungary Slav and German auxiliaries are also recorded, and Pechenegs were employed after 1051; there were quite probably many Slavs in the army at Lechfield, which would help explain its size”
So that’s where ther Slav infantry come from; they are indeed auxiliaries.
Where exactly were they recorded? There is a very rich bibliography about 10th century Hungarian warfare (a dozen of them on my bookshelf too), and it never mentions any auxiliary infantry- knowing the tactics, the society and the general scene, it would be like heavy armoured knights in an early republican Roman army.
The army sizes of the 955 battle at Lechfield (in Hungarian literature: Augsburg) is a debated topic, but the original estimation of 100k Hungarians is pretty sure just a wild exaggeration so common in medieval sources. The real number is more like 10k Hungarians and a slightly smaller German army (who had some Czech contingents too indeed.) The Hungarian army was led by the Hungarian Lords Taksony, Bulcsú, Súr, and Lehel, the latter commanding the auxiliary contingent. But this had no Slav infantry, it was also a horse archer force, mostly of Kabars, employed as a vanguard contingent, which was a common usage of auxiliaries in nomadic steppe nations (also later in the Kingdom of Hungary). Kabars or Kavars were a nation of Turkish origin, they rebelled against the Kazar Empire and their 3 tribes joined the Hungarians before the occupation of Hungary. They settled down in Nyitra, in Northwest Hungary (today belongs to Slovakia), and melted into the Hungarians not long after. During the late 9-10th century they were the usual auxiliaries of the Hungarian armies.