GAC - there are no general rules that say institutions can not invest in companies with debt or long term debt. The mandates of each fund is what determens what types of investments they can make. Many institutions mandates state that they can not invest in companies "with no revenue" that carry debt. This would be similar to what many estate trustee's of high net worth individuals have to abide by as well because of clauses that were stipulated in wills. I don't think you will find any hard fast rules, if so it may be found at the public trustees office in some kind of list of eligible investments.
Good luck.
tada