Regarding your questions, the first question, when a soul reincarnates, does he associate with the souls he associated with in his lives, that is not necessarily so. Even if he associates, how can he recognize them because everyone changes his body. Regarding your question about karmic debt, yes, it is generally that the debtor is obliged to take birth and also the creditor. Sometimes the creditor takes birth as the son of the debtor and after being a very affectionate son for a few days, he dies and thus the debtor becomes aggrieved very much. This is the punishment. In this way in every transaction the participants are becoming involved in their resultant action of karma. This is karma-bandhana, in Bhagavad-gita, or the bondage of different fruitive activity. It is advised that one should act only for Krsna, otherwise he will be involved in karma-bandhana.
Regarding your second question, whether a person who is a Negro, Chinese, Indian, etc. are they different species of life making up the 400,000 species. Yes, so far their body is concerned. Your question whether woman in each one of these species is another separate species, no, the species means both man and woman of the same type. Of course, strictly speaking the woman is taken differently, otherwise how would Krsna say striyo vaisyas tatha sudras.